For example I have a nested label "Invites" in the label "Calendar" (Calendar/Invites). Searching "label:calendar-invites blah" searches for "blah" inside the nested label.
However, to perform a successful search with specific constraints, you must either:
1) Remember a keyword
2) Have a label for the particular constraint you want to use
Our little startup, SolNovus (http://solnovus.com), has been investigating how to make a couple power-user functions like this more accessible. Our priorities are:
1) Fuzzy topic search: make keywords more forgiving
2) Analysis and visualization of email graph: sift through related threads and see the impact of search constraints, represent scale and organization with more than an inbox counter
The issue arises if you know the message you want is somewhere under Calendar, but you're not sure whether it's in Calendar/Invites or Calendar/Rejected or Calendar/DateNotSetYet. As far as I know, it's not possible to search them all at once, just by searching under the tag Calendar.