Janine,
I know it works in 4.6 tip because that's where I was working until just a few weeks ago. My notes indicate that I added callbacks (using 4.6) to a package in September, but I'm fairly certain that it was actually prior to that. Don't know how close that was to 4.6.3.
If you turn debug on and install/mount a package (say acs-subsite), I believe you should see apm log messages when it invokes a callback.
I'm looking at 5.0 api-doc and I see that you can exercise the callback mechanism through the apm api apm_invoke_callback_proc (and others). In fact, you can set/remove/get callback procs with the api. Worth a try...
Randy
