I noticed that there are actually phosphor bronze electrodes for SMAW.
According to the datasheet (http://tinyurl.com/dhx5u), these rods could
be used to weld copper. Has anyone used these? In any case, I'm sure I
would have to buy a minimum of 10lb and I don't need that much.

I think I'm going to try brazing, though.

I have some Stay-Silv 56 from another project. Would that be a good
alloy? Or should I use their phosphor copper alloy? I guess one would
use the phosphor copper for better color matching? However, using the
staysilv 56, I would only have to get the copper to 1200F instead of
1400F.

If I braze the copper bars, would a mitre joint be the appropriate
joint? It seems there would be more surface area than a butt joint, but
it still seems like there should be more of a mechanical fitup for
brazing.