Or rather, was it safe? I mean... I probably should have asked before actually doing so... :-P
My understanding was that on FreeBSD systems, system stuff goes in /usr, while userland stuff goes into /usr/local.
The Mac still boots just fine and seems happy, but I'm getting excruciating difficulties reinstalling things using homebrew in the aftermath... hence my wondering... Was it actually safe, or am I heading for a complete OSX reinstall?