For now, it IS possible to search on the author or the commit message in the merge dialog. It would be very helpful if it was possible to search as well on the commit number.
Regards
Syncro SVN Client 3.1, build 2008030417 (trial version) Mac OS X 10.5.2 (9C7010)
We already added this feature request to our internal issues tracker and will consider implemented it in a future version. We have no definite timeline for this but you will be notified when this feature becomes available.