I don't know much about plating but is hydrogen embrittlement a concern with nickel plating? The Wikipedia entry says "Steel with an ultimate tensile strength of less than 1000 MPa or hardness of less than 30 HRC are not generally considered susceptible to hydrogen embrittlement" - I guess this would mean most of the steels used for Locosts would be safe?
Hydrogen embrittlement is not a concern with the mild and low carbon steel used in a locost. Also not a concern in plating as the hydrogen enters through molten metal during welding, and then diffuses to areas of high stress. This can cause cracking in materials with high tensile strengths and higher hardness. If you do weld a higher strength or hardness steel as long as you store your welding wire the proper way and clean the area before welding you will be safe. Also cracking can occur anytime 48 hours after the weld occurred and in order for a post weld heat treat "post bake" to remove hydrogen you must never let the material temp drop bellow 300 degrees after welding and before heat treat.