Alright, I think this is safe to rule as invalid because the cause is most likely hardware failure: Oneiric boot media now also fails to boot here despite being the one used to install this system initially. I even went so far back as installing Jaunty (didn’t have anything more recent at hand, before Oneiric) and it also segfaults when X should start (though the graphical installer did work). I’ll still do tests if @Joseph disagrees (and has new ideas to test), but for now I’ll just settle for using fbdev (which seems to not trigger the issue).