So, it turned out fixing the Workbench crash on Snow Leopard wasn’t very hard. Took a little time to find out what was causing the crash but, once that was found, the fix was quick.
The next releases of both branches — MySQL Workbench 5.1.18 this week and 5.2.3 alpha later on — will have proper support for Mac OS X 10.6
For whoever might be interested, the bug was caused by what seems to be a slight change in how object/NSView copies are handed in Cocoa. NSCollectionView items are populated by copying a “prototype” object. It seems that somehow, duplication of that object included the fields of child objects as well in Leopard, but not in Snow Leopard. The object copies were incomplete and when they were accessed, it ended up attempting to create a C++ string out of nil.