It's an interesting project. I grabbed it and wrote a quick Flickr FS. It seems that it's a little limited for really laggy webservices in that you specify the type of an entry (directory, folder) by the type of data you return, and it calls the same method for readdir, getattr, and read, with no way to differentiate in your code.

In order to specify the type of an entry, you have to return the data for that entry. So a sub-folder has to return an array. There's no way to differentiate between, say, an ls on the parent folder or on the sub folder (e.g. ls / and ls /foo both return the method mapped to /foo), so you have to query all of the subfolders' contents at once AND cache it so it doesn't have to be re-queried when the user wants to look at the sub-folder.

Hopefully I'm overlooking something, but the source is pretty straightforward. The good part is it'd be easy to modify to ask for types separately. Actually just passing in another argument indicating what mode it's in would help.