We take advantage of the fogbugz/kiln integration by using "Case XXX" in all of our Hg/Kiln commits, so that when we look at a fogbugz case, we can see all of the changesets that belong to a specific case.
Fogbugz has no way of searching for a range of changesets, much less a single changeset What is needed is a "reveset" function and harness the power of Hg's query language. Here are two simple examples:
revset[ABCDEFG::] would return all FB cases with changeset ABCDEFG
revset[::beta and not ::gamma] would return all FB cases that exist in the beta branch and not in the gamma branch. This shows you all cases that have code changes between two different branches. This let's you find out what is documented in Fogbugz between two different heads.
This shouldn't be that difficult to implement. The native revset functionality is built into Hg see hg help revsets and all that the FB filter would need to do would be to search FB cases for the returned list of changesets.