Are we over thinking this? I have registered a lot of vehicles here in CA and have never once seen any of the DMV staff even try and look for an ID number anywhere but the door or dash.
I have a friend that buys salvaged vehicles and swaps frames and half frames all of the time and he has never seen CA DMV look anywhere but the easy locations.

I say you play dumb show them the door tag and say as far as I know it has the original frame. Or get it registered before the fram swap and never tell.
What they don't know won't hurt them.

jd