Has anyone had any experience piping words with chocolate? Any tips? At this stage, my plan was to put some chocolate buttons in a zip-lock bag, tape it into a piping bag shape, melt in the microwave, then nick the corner off.

I was just going to do the letters the same as with royal icing, but is there anything I need to know?

T.I.A.!

I've done it with candy melts (I don't know if that's the same as chocolate buttons or not, but it's probably close) and it's not that different from piping with anything else, except that you have to keep the stuff really warm, or it doesn't flow well. I'm usually trying to fill in larger areas, though; if you're piping thinner lines (like cursive writing) then you probably want it not quite so warm, so it won't spread and flatten too much.