a work permit is not a visa. It’s in addition to a specific few types of visas. And DTV is employment prohibited.
In the real world though, if you work with a reputable charity , they will have an understanding with local law enforcement, government, politicians etc. and it will be fine.
They ban exists because of too many people trying to cheat the system
I tried to get on a tourist visa, but they said no. But they did say that if I come with a DTV they would allow it. But, I haven’t actually put it to the test yet.