Emacs documentation states that you can use query-replace on filesets to perform find/replace on multiple files.

This lets you perform certain operations, such as visiting, query-replace, and shell commands on all the files at once.

I've opened my particular fileset (i.e. via fileset-open) and attempted query/replace using both:

M-%

and

M-x query-replace

but can't get it to work on anything but the file (buffer) I'm currently in.