The top answer is:
If, on a given site, there's a big problem with runaway tagging... Post a feature-request on your meta along with examples that demonstrate the need. We've temporarily lowered this in the past for sites with a lot of cleanup to be done, and I think doing this as part of a larger, coordinated cleanup effort is a good idea. Don't forget: even after a synonym has been created, the questions involved still need to be retagged, either by individual editors, or by a moderator merging the tags.
I was going through @tchrist's excellent list of synonym suggestions but couldn't propose most of them because I didn't have the required rep counts. Seeing that many of the tags are so infrequently used, it seems unlikely I'll get the rep any time soon.
Thus, this is a feature request to let trusted users submit tag synonyms. A trusted user is anyone with 20k rep or higher.
EDIT: Since no one has responded to this feature request, can someone at least offer a suitable workaround? The above linked posts suggest that this is the appropriate thing to request.