Tesla made a similar adapter for a couple years after the Model S came out that went from Roadster to Model S. You could then stack an adapter that went Model S to J1772. You can't get it from Tesla anymore but once in a while one pops up on ebay or the Marketplace section of TMC.
I thought about that, but I was hoping to avoid stacking adapters for cost and complexity reasons.
What is the Roadster charging rate currently? If you're hard-wired into a 50 amp circuit, it might be easier to convert the charger and wiring to a 14-50 outlet and let the visitor charge from there (unplugging the Roadster's charger). Or, just add a 14-50 outlet to the garage. It's kind of the universal home EV charging outlet, and can be used for other things (e.g. RVs).
I do have a 14-50R available, but I'm finding that my friends who drive other EV brands tend to not have portable Level 2 chargers. They seem to have non-portable L2 chargers in their garage and the Level 1 (120V 12A) charger that comes included with the car. I have found that the Level 1 chargers that come with the Chevy Bolt/Volt and Fiat 500e will happily handle 240V 12A so I bought a 14-60/14-50/14-30 to 6-15/6-20 extension cord and a 6-15 to "Worldwide" travel adapter to put on the end of it. This way, my visitors can plug their L1 chargers into my NEMA 14-30 dryer outlet and charge at 240V 12A, which isn't too bad. (that travel adapter is rated for 10A so I'll have to keep an eye on it to make sure it doesn't get hot).
That travel adapter was about $5 from some random vendor on amazon, and that very nice extension cord was $70 from EVSEadapters.com. I have nothing but good things to say about them, they have lots of adapters, cords, and chargers and even made a custom extension cord for me so I can plug my UMC into old dryer outlets 50' away.
Ultimately, I suppose the best solution would be to just buy a 32A J1772 portable charger for visitors.