I ended up using XTC-3D to seal the mold (printed using Shapeways strong & flexible plastic), and Smooth-Sil 940 to cast the final chocolate mold from that. I haven’t yet had a chance to temper and pour some chocolate, but I’ll post an update when I do.
A few things:
1) The XTC-3D does pool a bit in the nooks and crannies of your model, so I suggest following up with a dry brush or sponge to mop up excess. You can also dilute the XTC-3D using acetone if you want a thinner coat – I haven't tried this, and it lengthens the cure time.
2) The detail on the model will transfer almost perfectly to the silicone. This includes the finish. So, if you have a nice shiny finish on your model then you will have a similarly nice, shiny finish on the silicone mold. I mention this because if you choose to sand the mold using fine-grit sandpaper after applying your first coat of XTC-3D (I used 400 grit), you should follow it up with another thin coat to get a uniform shiny surface. I did not do this, so there are patches of matte finish where I sanded mixed with glossy where I didn’t. It doesn’t look bad, but it could be better.
3) I did not use any releasing agent on the surface of the model before pouring the silicone, and that ended up working out fine. However, if I’d thought ahead, I would have designed my model a bit more wisely to help out: angled instead of vertical sidewalls on the lettering, and a lower aspect ratio in areas where spacing gets tight (e.g. between the bottom of the “J” and the boundary).