Ok, I asked ruby-core about release cycles, and they're preparing to spin a new 1.9.3 release soon. I'm not sure how soon that is, but it will certainly be before JRuby 1.7 final.
I have also asked if it would be safe for us to roll to the most current stdlib from svn today, assuming the released version will be at least this patchlevel. So the problem is going to solve itself shortly, and after that we'll try to make sure we're matching patchlevel.
FWIW, the main reason I had been keeping up with svn rather than actual releases was to pick up bug fixes that were taking too long to reach the public. Obviously we could have been picking up bugs at the same time.