Good question!
While in general, the answer is that Boa does not (explicitly) contain a list of repository tags, at least for now Boa only has CVS and SVN style repository data and for those repositories (because CVS is converted into SVN) you can find tags using the standard SVN directory layout.
This layout says that all tags are in '/tags/.../'. You can see an example of a query looking for those here:
The output here is indexed first by project ID, then by the tag. The counts on the right are how many ChangedFiles have that tag.
- Robert