I "modelled" five unzipping stages for the V3 Nancy Boots using the cloth room, then moved the zip pull itself to the right place for each stage.

However I couldn't make it work as a single morph, because it would have to follow the curve of Victoria's leg and, as noted above, morphs move only in straight lines. The options here were to have five morphs and only use one at a time, or go for geometry switching. In the end I settled for the latter, but that's a whole new can of rigging worms. ;)