Thanks for the positive reference, but please understand there are many on this forum that have a much better grasp of much of this nix stuff than I do (I consider myself to be "just a little less confused" than some on this stuff, and actually hope that some of the *nix "wizards" out there will jump in an do a better job of sorting through this than I can). It looks like you have "researched" quite a bit on this, and I can see how you are thinking, but I *think you are overlooking one thing:
The "re-mapping" the sub-directory accomplishes making the content in the "other" user's directory available as a sub-directory of your domain via http. The way it does this is dependent on the apache server, and FTP is an entirely different protocol.
While it might seem like a subtle difference, you may notice that the wiki article on re-mapping a sub-directory only represents that these files will be accessible via a browser at the yourdomain/subdir remapped to, and cautions you that there will be permission issues.
In short, it "re-maps" http, not ftp. This is probably "just as well", as if you do what you are attempting, you will have to deal with/account for permissions differences, and that can get to be a pain.
Does that help at all?
I actually have several clients with the same situation you describe, and what has worked best for me is simply providing them with a "filemanager" cgi, (I suppose if you have read my other threads this is starting to sound like a broken record!) set up in a subdirectory of a site, protected by .htaccess from casual borrowsing by others, and/or password protection within the program.
This is "better" to my way of thinking because I don't have to worry about the users learning/borking the use of an ftp client, I end up the nix "owner" of *all the files, and it is "easy" for them to use and for me to set up.
I recommend a perl program for this, as there are no problems with PHP's filesize lilmits for uploads. The "cleanest/simplest" program I have found for this is ffileman v.08, because it is so simple for me to install and for the users to use (though there are others that work just as well - check hotscripts.com for more chioces than you will want to wade through!)
I'm sorry if this doesn't make any sense, but I'm at a loss how to explain it better. Please don't be offended if I seem to be dismissing your question and just suggesting you take another approach, becauses that is not my intention. I am trying to show you a way to accomplish what you want to do with "less grief", but I am not meaning to represent that "my way is the only way". YMMV