Norwegian Krags, and perhaps Danish also, have their single front bolt lug AND the lug formed by the rear of the bolt guide [lug] as bearing surfaces while U.S. Krags engage only the single front bolt lug.

During a rebarreling and general truing/blueprinting of any U.S. Krag action (1894, 1896, 1898) can the gunsmith create that second bearing surface at rear of bolt guide-to-receiver bridge without a great deal of trouble and expense?

The key phrase in your query is the last one, regarding trouble and expense.

Basically, 3 ways: grind the front bolt lug, weld the rear bolt lug, or weld the front of the receiver's rear bridge. The gap will usually be somewhere in the 0.005"-0.010" range.

I've never done this alteration but Michael Petrov in another post has said that the case-hardening was largely gone after grinding 0.005" off the rear of the front bolt lug. IMO this is unacceptable without rehardening.

For the same reason I'm reluctant to weld on the bolt's rear lug. The welding is child's play but what about the rehardening?

The remaining alternative, welding the right front of the rear bridge to build it up, in my mind is preferable over the other 2 choices. It's relatively easy to reharden the localized area where the weld has added material to the right side of the bridge, but there's always a risk of warpage. Not much of a problem to a smith; a reheating and/or a lapping will solve the binding if any.

FWIW I'm building a Krag right now and am not going to worry about this 'problem'. Supposedly having 2 lugs makes the rifle more accurate in the rain such as in match shooting or military acceptance tests, but I'm not gonna sweat it.
Regards, Joe

I have read that Sedgley (1920's) used to
modify his Krag sporters so the bolt was touchung. How he did it ?. He did proof them
with a 30/06. How he did that ?.