I agree with Mike Hall's assessment that the parts are from China, and Microsoft MTL has no individual parts in store. If anything under warranty is repaired and not merely replaced (as is usually the case on items like keyboards and mice (mouses?)), it's handled by some third party organization, not MS.
If you know where the problem is (broken trace), there are trace repair products on the market. The ones I've used are a fairly thick "goop" with silver dust in them that were applied with a small pointed brush. Of course, this is assuming that the traces are far enough apart to allow a thin line of the material to be applied without touching another trace. Maybe protect the other traces with something like clear fingernail polish first (better test that on an unused portion of the film first)? I don't know, but if you're game to try something like that, it might be worth a shot, if the cost of repair materials doesn't outweigh the cost of a new keyboard :-)
SC Tom