Maybe it would be a bit harder than I thought to make it backwards compatible. Keep thinking about it though.
The reason I asked for it in the first place is that I was making a script to insert filenames into textmate, a bit like pressing tab in terminal. Sometimes I would get more than one result, and I wanted those to appear in a list. Now I've been thinking about it though, it might be a nice thing to have this behaviour built into Textmate.
An example would be that if you typed in an html document: "href='i" and then pressed tab, you could invoke a menu which showed files matching 'i.*'. It could even be multi-dimensional as in my included mock-up.
Thamks Max