comment-XPath-8 XPath

but it still constrains operations to one snip at a time

No, you can use several doc() and join them but you have to write the names of the snips.

I can't use an XQuery to find, for example, all snips with a date label with this approach

I think I have to do some thinking how this could be solved.

update

1. Use lucene search in url to a servlet returning a list of matching snip names which you can use in a loop and read all content by doc(.../$name) example with a label: ...?query=xmppID:senikk*
2. Make a servlet returning all snipsnap names and labels as a xml document