Actually, it wouldn't be a big problem to do this. Sure, you might produce what you thought was a normal, but was actually het hypo, but who cares?
The bigger problem is if you do it in reverse order. If you breed the pewter to a normal the first year, and the next year breed that same normal female to a hypo. If she retained sperm from the pewter, you could produce normal offspring that were sired by the pewter and truly are normal and not het for anything, yet you think they were sired by the hypo and that they are 100% hets.