Allow way to set mapping rollback_action when creating stubs

I'm encountering an issue I'm not sure how to work around. Here's the scenario.

I have two migrations, one which imports user pictures, and another which imports users. The UserMigration depends on UserPictureMigration. But since I need to import UserPictureMigration first, I've implemented a createStub() function in UserMigration.

In that stub function, I check to see if a user exists by the ID (which in this case is email). If so, I return the existing UID. Otherwise, I create a new stub user and return its UID.

This creates a mapping record which always has rollback_action = 0. However, I want to prevent deleting certain existing users (like site admins) when rolling back my UserMigration.

I cannot find a good way to do this. In Migration::createStubWrapper(), where the stub function is called, an ID mapping is created, which uses $this->defaultRollbackAction to determine what rollback action should be associated with that mapping.

Is there a way for me to override this somehow during the stub creation process? One idea I had was in UserMigration to set the defaultRollbackAction to MigrateMap::ROLLBACK_PRESERVE, but then all the stub'd users get that value. I can override it per user in prepareRow(), but that requires me to run the UserMigration before I rollback UserPictureMigration, which means I have to also rollback UserMigration.

Any ideas?