Okay, barring any device surfacing to handle this, I'm going ahead with some research of my own. Apparently one reason devices and information are not widely available is that Apple may have a patent on some forms of headset commands through the TRRS 3.5 or 2.5 mm jack. Since I'm a new forum user I can't post any links out right now; sorry.
I have a SkullCandy headphones+mic. They have a single button next to the mic. This button works to pause/play the Meridian music player on my Droid Incredible. Pushing the button twice rapidly pauses and then skips to the next track and plays. If I push the button when no music player is open, it opens Meridian and plays. If I push when some other music player is open, it opens Meridian and plays.
Presumably the button works by changing the resistance over the headphone wires. Checking the resistance between the sleeve (which seems to be ground) and the other contacts (Tip, Ring1, Ring2) I find that without the button pressed, the resistance is about 1 kOhm for all three. Pushing the button changes the resistances to be: 17 Ohm, 17 Ohm, 1 Ohm for the Tip, Ring1, Ring2 respectively.
So if I can figure out what the head unit sends to the CD changer when the radio buttons are pushed, I may be able to rig up something useful. I am still curious what other signals might be possible. When I have the shorting out going crazy behavior there definitely seems to be a case where it opens the voice dialer window or calls the last number in the call list. Not clear whether this is a different hardware signal or something about the software state of the phone when it gets the resistance change. Haven't yet tested what happens when you push the button and there is a call, but presumably it answers/hangs up. There are other headsets (for Apple players mostly) that have additional buttons too; not sure if that's relevant.