Interesting concept. I've never done it (or had reason to) but I am aware of some smiths using this technique for filling voids. You are right, it would take considerable skill to do invisibly, but not impossible! I believe I'd use a judicious application of low-temp silver solder to safeguard "the patch" working loose, possibly losing the numbers. With the possible exception of very high end guns it seems a lot of unnecessary trouble though... just my humble opinion.