Try using ISheetMetalFeatureData::AccessSelections immediately after calling IFeature::GetDefinition. Technically AccessSelections is only supposed to necessary when you're modifying the selections used to create the feautre, but more quite often I've seen it necessary to use this method to get other properties to stick as well. AccessSelections is what puts your feature in a rollback state for editing.
Thanks keith,SheetMetalFeatureData::AccessSelections actually didn't end up being nessecary, but after including it I was searching for a way to "roll to end" after making the property change. This led me to SheetMetalFeatureData::ModifyDefinition which ended up being the missing link. It looks like I might as well include the accessSelections though, just for good measure.