Everything depends on how deep the pits are.  I wouldn't worry about case hardening, but if you have to file/grind/polish deep enough you could weaken that very critical area of the rifle.

I know of no filler/solder/epoxy, etc... that bluing will take to.

A couple of things to consider:  A trick I saw once on an 03 with pits--gunsmith used a center punch and hammer to dimple the pitted receiver ring.  He put small dimples all over the ring and this effectively hid the pits and gave the rifle a very unique (and quite attractive) appearance once it was blued.  Another thought is--bluing was not the original finish on Krags, the receivers were a dull grey color caused by oil hardening.  You could always fill the pits with epoxy and paint the receiver.  Yes, I said paint. Paint on a rifle may sound like Bubba, but that is the finish that is applied on some rifles from the factory now.