Update: it seems to have worked!! Substantial activity from the airlock. I chilled the water bath a little too hard to start (down to 52F) but chugging along nicely now at 62F. FYI for anyone else trying this, it was actually a pint of finished wort, hops and all, that I diluted down to just over a quart, 1.052 down to 1.033. Thanks for your help, everyone!