The M2 submits
As I have previously documented, I've been setting up a Sun X2100 M2.
It's not been the smoothest ride. And I wouldn't say that everything is completely solved, but having got it installed I followed the instructions for serial redirection and I can now ssh to the SP and get console access. That's probably all I need, although I still have a slight lack of confidence in the system - I can imagine ways in which it could get itself messed up where I get no access, but for most of the administration tasks I'll ever need to do I'm sorted.
Still can't get Serial-over-LAN to work, though. Never mind, ssh is good enough for me.