For closure:

You can build a bitcoind (headless bitcoin) on a Mac. The build instructions are not with the usual binary distribution but you can grab them off the github repository for bitcoin. The minimal dependencies are: boost (from boost.org) and Berkeley DB (from the freshmeat.net project), you (apparently) can use libssl and libcrypto from the system.

bitcoind appears to run undaemonized, so it should be suitable for a launchd script. But I haven't tested this configuration yet.

Cheers,
G.