I'm in the same boat as you. I'm a telecommuter, so all my projects are three timezones away on an ftp server.
I would think that the issue isn't so much technical as time sensitive. To search in the files, most editors open a copy of the file in memory. This means that phpEd would have to transfer every file via ftp, then open it in memory to do the find. There are ways to do meta-indexing (think Spotlight from OSX). Perhaps save the index in the project file? When you create a project off of an ftp directory, there would be a bit of overhead time as phpEd indexed the contents and did all the code evaluation (for code completion, etc).
Of course the
REAL answer to this is version control. Local sandbox - remote checkin. Let your svn or cvs client deal with the up and down. You have the local copy to play with, and you can do a true "Smart Sync"
with the remote box.
-Mark