I am wondering if having an action heat treated (recarburized if you will) can distort engraving. Or in other words, if you want to have a custom rifle made from say a mauser action and you want some engraving on the receiver, should you have the receiver engraved first before its recarburized (so that the engraver doesn't have a tough time engraving it) of should you wait to have it engraved because the recarburization will distort it.

I'd ask that question directly to scrollcutter Via PM.....My guess is that you'll get the very best reply from him

I'll let one of the engravers respond but from my heat treat experiences all parts will develop a bit of scale on them. Depending on how the part was protected in the furnace will determine how much scale is built up on the part.
It would be my guess that engraving would suffer.
And engravers can at their option use carbide gravers that allows them to cut hardened materials but there are some materials just too hard to engrave.