If you're really desperate, do a dump of the repo (look at "svnadmin dump") and then grep through it. It's not pretty, but you can look around the search results to find the metadata that indicates the file and revision, then check it out for a better look.
Not a good solution, to be sure, but it is free :) SVN provides no feature for searching past checkins (or even past log files, AFAIK).
