Somehow I just now noticed the followups to this thread, sorry for the late response. I would like to keep the thread alive though.
I was hoping to have that done and submit the immutable-systems patch before the next major release, but obviously I'm too late for that now -- I happen to be starting the Windows tests for 3.1.4.13 right now, which are apparently the last hurdle for a 3.1.5 release, and I certainly don't want to suggest holding up the 3.1.5 release by trying to cram the immutable-systems patch into it. After 3.1.5 is out the door I will attempt it during the subsequent window of opportunity.
I admit I've been shipping my immutable-systems patch privately with the asdf shipped with Gendl and GDL pre-built distributions since last summer. This is obviously dangerous practice on our part and has to stop.