You already can, as I mentioned. /apps/banshee-1/library/folder_pattern
and file_pattern. And I think Banshee certainly wants to cater for
non-coders!
Well, in my case I don't like to put a period between the track number and the track title, and there's no option in the list for that, so I modified it with the GConf editor. Anyway, perhaps it would be nice to have some "Advanced" button besides the options list where all the %xxx% are listed and can be added.