I am involved in the OpenACS based project .LRN (see http://dotlrn.org
I can not address all your quesitons, but it sounds like one of your needs overlap with things that are being worked on for .LRN that will be flowing back into OpenACS over the next couple of months/weeks.
So let me take the first stab at your questions...
1) I have not heard of anyone doing it, but OF COURSE it is worthwhile (doubt you will get a different answer asking here)
2) Leaving #2 for someone else, but this would be a great contribution and it sounds like something that would draw collaborators quickly
3) There is work on site registration tools and data collection tools going on right now in OpenACS that will be used in .LRN to collect different kinds of data related to individual users and displayed based on roles within the system (e.g. in .LRN a course admin might want to ask some questions and sort users based on those questions, e.g. at the time of registration on a site most organizations want to collect unique user data and display it in different contexts in the future). The assessment package will be the data storage point for this kind of information. http://cvs.openacs.org/cvs/openacs-4/packages/assessment/www/doc/ is the source of the most recent documentation for that part of it.
4) OpenACS CMS (the package) is not being pursued right now in the community, but pieces key pieces of that package have been extracted/reworked for content management use cases. I am not on top of this part of the toolkit right now, but the last time I heard bcms/bcms-ui-base/bcms-ui-wizard (the combination of these packages) was the path the OpenACS community was going down. Maybe someone will fill us in on OpenACS cms tools in this thread?
5) Is there an upgrade path? I doubt it, but if you have a will I am sure there is a way. FYI: I have heard nothing (good) about the ACS Java version yet (other than management seems to like Java) and the last time I looked at the website for any signs of activity I ended up complaining to RedHat for including case studies that are actually running on OpenACS (they have taken them down in the meantime I assume). Update: You motivated me to take another look at it (it is hard not to tire of the Java line). It seems "byline" is what they call the ACS Java version now (the version Redhat is not making money on writing individual customizations I assume... i.e. maybe an alternative upgrade path if your management has the cash in the future?).
1. Not much on the applications side of things yet: http://wiki.objectweb.org/byline/Wiki.jsp?page=Applications so upgrading any time soon might be hard.
2. They are pretty bug free: they only have two open bugs and a single closed bug (although they do point to redhats bugzilla for "core bugs" rather than this third party project hosting solution that they are using)
3. There is not as much going on in there forums (12 posts since September?)
If you want to chat with me about byline any more you can join their freenode IRC channel (I would appreciate it... I am all by-myself on the channel they list on their site right now)
My synopsis for your management: the people using and creating the software is what makes or breaks it, not the language (and things are going well in our universe).
On behalf of the OpenACS community: welcome (or good luck finding something equivalent)