How'd I miss these???
We'd only need that if we supported linking with the libmysqld embedded server, which we most definitely do not right now.
Instead, wrap janky signal-unsafe MySQL API calls with signal block/unblock actions at the C level. This should be faster than using bound threads, and also avoids the perils of running out of OS threads documented in http://hackage.haskell.org/trac/ghc/ticket/5174