Interesting problem so I'll put my two sense worth in. And since I've never confronted this before, all I can do is brainstorm.
First of all what type of weld and what type of repair are you in need of?
I would try and find a way to either machine or grind the nitrided layer off. If not, that much nitrogen available I believe may cause porosity on the weld, and it will cause your microstructure to be fully austenitic and therefore prone to hot cracking.
And since the usual weld material for this is 347 you will also be faced with the CbN problem which may intensify any hot crack tendency.
Just as a seat of the pants consideration I would think that you would have little problem, beyhond those normally associated with fully austentic SS welds,  if you can remove the nitrided layer. I could be wrong of course. But even at this you will need to ensure minimum design thickness and therefore will have the toe of the weld still influenced by nitriding.