Cutting this bar is never fun. I've thought about making a chocolate
cutter out of a piano wire or maybe a guitar string - sort of like the
big cheese cutters they use at the cheese store that I frequent. If I
could heat the wire, it would make the job much easier. Is there a
safe way to electrically heat such a wire sufficiently to cut
chocolate? What sort of low-voltage circuit could I use for this? Or
is this idea completely off the wall?

Don't make this into a project, unless you are bored. Simply heat a knife
over your stove and then slice the bar.