You're saying that you have a properly installed "to code" NEMA 14-30R. It should have 10AWG wire and a 30A circuit breaker. The circuit breaker protects the wire, so the size of the circuit breaker needs to match the size of the wire installed. You replace the 14-30R with a 14-50R on a "temporary" basis. You don't need to add another inline fuse or breaker "near" the 14-50R; it and the wire will still be protected by the original 30A breaker. Try to draw 40A, and the feed breaker will pop.
In another thread, he revealed that he had a Zinsco circuit panel. Given the design flaws and failure rate of Zinsco panels and breakers, you must assume he has no over-current protection because those breakers have a high rate of failure-to-trip. He could use a subpanel and/or inline fuse -- some other OCPD -- but it still would not be to NEC (if installed as infrastructure wiring) and/or would not be listed (if installed as an extension cord).
Read your insurance policy. Mine has no such exclusion for use of a non-UL listed device.
Note that I said "violating the NEC with a non-listed device", not just use of a non-UL listed device.
I should probably be more specific but I was short-cutting a few things, so the way I communicated it could have been misinterpreted. It is not that it must be listed, but rather that listing is generally accepted as compliance with current NEC requirements and is readily accepted as such. If a device is not UL-listed, but still complies with the NEC as adopted by the AHJ, it is still legal, and insurance will cover that. If a device is UL listed, but is used for a non-listed means, insurance may not cover that.
The kind of stuff (FrankenCables) we're talking about here is plainly in violation of the NEC, though -- if you cut the prong on the 14-50 so that it will fit in a 14-30, you've modified the device and are violating the device's nameplate ratings; if you create a 14-30P to 14-50R adapter cord, you've violated the NEC provision prohibiting smaller receptacle ratings on a larger-rated branch circuit.
Every insurance company is different, of course, and if you pay more you likely have more coverage. I can tell you, though, that I have seen homeowners lose on an insurance claim because they violated the NEC with some rigged circuit extensions. Since they'll be the ones paying if you lose, check with them.
I have State Farm, and I looked up the provisions of my policy. You could be denied coverage under the following provision(s) if you have their coverage:
2. We do not insure under any coverage for any loss which
would not have occurred in the absence of one or more
of the following excluded events. We do not insure for
such loss regardless of: (a) the cause of the excluded
event; or (b) other causes of the loss; or (c) whether other
causes acted concurrently or in any sequence with the
excluded event to produce the loss; or (d) whether the
event occurs suddenly or gradually, involves isolated or
widespread damage, arises from natural or external
forces, or occurs as a result of any combination of these:
a. Ordinance or Law, meaning enforcement of any
ordinance or law regulating the construction, repair
or demolition of a building or other structure.
[...]
d. Neglect, meaning neglect of the insured to use all
reasonable means to save and preserve property at
and after the time of a loss, or when property is
endangered.
[...]
3. We do not insure under any coverage for any loss consisting
of one or more of the items below. Further, we do
not insure for loss described in paragraphs 1. and 2.
immediately above regardless of whether one or more of
the following: (a) directly or indirectly cause, contribute to
or aggravate the loss; or (b) occur before, at the same
time, or after the loss or any other cause of the loss:
a. conduct, act, failure to act, or decision of any person,
group, organization or governmental body whether
intentional, wrongful, negligent, or without fault;
b. defect, weakness, inadequacy, fault or unsoundness
in:
[...]
(2) design, specifications, workmanship, construction, grading, compaction;
(3) materials used in construction or repair; or
[...]
of any property (including land, structures, or improvements of any kind) whether on or off the residence premises;
I have seen this happen, unfortunately.
It better not. Your Tesla charge cord, when used with 220V (or 110V > 20A), violated NEC 625 (1999). NEC 625 (1999) requires that the EVSE (EV Service Equipment) for 240VAC be permanently connected (i.e., not plugged into a 14-50R). Later versions of the NEC allow certain UL listed cords to be used that aren't hard-wired, so you would need to know which revision of the NEC that the place you're charging at has adopted. (I also believe that their use is restricted to "indoors", so the use of a 14-50R at a mobile home park, eg, would violate the terms of the UL listing and hence the NEC.)
1999 is quite old and most AHJ's have adopted NEC 2005 at a minimum, if not 2008. However, NEC 1999 does indeed permit more than 120v, 15/20A (I dug up my old code book for this one!)... 625.13 allows for "...or part of a system identified and listed as suitable for the purpose and meeting the requirements of 625.18, 625.19, and 625.29 shall be permitted to be cord-and-plug-connected". 625.18 requires an interlock to make sure the power isn't on when the connector isn't plugged in (UMC does this through its contactor); 625.19 requires de-energization of the cable that would be caused by cable strain (UMC does this through its contactor); and 625.29 specifies "indoor sites" only (to include garages, parking structures, commercial garages, and agricultural buildings).
Then again, you're getting into gray area that is the partnership of the NEC and listing agencies and the AHJ. NEC typically governs the infrastructure, while UL listing governs devices that make up and use the infrastructure. And the AHJ has the final say during inspection and can give written authorization to violate both. Many AHJ's recognize the "new" nature of some equipment, and that is why AHJ's will typically sign off immediately when a listed device is used per the manufacturer's instructions.