I want to create a "File Open" dialog that's restricted (or jailed) to a specific UNC (\\tsclient\). When opening files, users should only be able to see folders above this point, and they should only be able to enter filenames which belong above this point.

browsing and opening \\tsclient\c is OK
browsing and opening c:\Windows\ is not OK

Has anyone done anything like this? Is it even possible?
I'm loathe to create my own File Open Dialog when the standard dialog may offer this as an option.