It might be simpler to use the adapter that already has the right resistance and construct a short adapter cable with a 30 amp recepticle that plugs into a 50 amp outlet. Not quite as elegant as your proposed approach, but probably easier.
The challenge is that many of Teslas adapters are vanishing slowly from their catalogue, it appears Tesla has taken the stance that Supercharger, chademo, J1772, HPWC, 14-50, and 5-15 are all anyone will ever need.
Right now 5-20 and 10-30 are still available in their store, but several other adapters have preceded them out of stock permanently, and I wouldn't be surprised if those follow shortly.
My most likely project though would be to take dual 5-20 (or 5-15) outlets and convert to a single 240v 20a (or 15a) supply. I'm still not sure what Tesla's adapter will do there... (most others in the past have wired that to a 14-50 and manually limited the charge in the car, I wonder if I could tell it 15 or 20 amps via a resistor instead? )
The other factor is, any of these adapters leave a risk of someone plugging in something that isn't expecting the item to behave that way, I'd rather skip that. (ie, if I make an adapter that takes 2 5-20 outlets and converts it to a single 5-20 outlet to take advantage of the 20a adapter that Tesla already has, I have now created a 5-20 outlet with 240v on it, that's just asking for trouble! I'd rather that all standard outlets put out their expected ratings, and anything that is non-standard is some form of connector that has very little risk of someone using by accident.
