Don't know if it'd me hard to code, but oh well, I'll suggest it anyways:

/pervisit [mapname] [name]

It'd only let the person which you put in the [name].

For example: /pervisit xerox218 zionfox

It'd only let Zion get into my Map, no one else would be allowed to do so, so only Zion could build in my map. This could be useful when you let someone of a Lower rank (For example guest rank) and they ask you if they can build in your map. You don't want any other guests getting in your map, just him.

I think you should understand what I mean

I could see it being easily done for the single user, in a one time basis, taking the variable out of memory once the player has joined your map, however it could cause a problem if the permissions needed to persist, causing for rewriting level properties files and making them very large for popular servers.