I've never tried planting using Tempest's Plant Here function on another residential lot as a guest. I've used it on community lots though, and the revision notes suggest it should work anywhere. Perhaps not as a guest though.

Traveler's 'treat as vacation' probably won't help then. A guest would still be a guest even if the sim isn't on vacation. I don't have any other suggestions.

I have to agree, you can't really do that without moving the sim into that world "permanently" with Change Hometown and then into the hosts' household. That does mean they will stand to lose their home and any other real estate in their original homeworld unless you have other household members back in it still living there.