The suggestions from UncleNick are probably the best way to go. The Sharpie, however permanent it's claimed to be, would fade and the oil might cause it to bleed and make the signature less sharply defined. Another option is to have one of the woodworking companies (Woodcraft or an equal) supply you with an iron brand that could be used to burn the signature into the wood. That approach isn't cheap, but would look very good if done properly. As with the other suggested approach, you'd send an example of the signature and what else you wanted, and they'd send you the brand (to be heated with a propane torch or heated electrically). It could be, however, that the size of the signature would be smaller than you would like. That is a limitation in doing it that way. A positive might be that Granny would then have something that she could use to brand anything that needed branding.