I'll start a full thread on this later but wanted to at least update here with pics of the end result for this model:
The plastic model from Shapeways:
The silicone mold cast form the plastic print (filled with shapecrete):
The finished cement casting (with crack right down the middle):
I ran into two issues with this project, 1) the model I sent to Shapeways was a TIN with triangular facets for the model surface, but the print I got back was smoothed like a true terrain. I'm still working out the what the issue was, it may be that I used too detailed a TIN (too small facets) and opted for a "polished" print which may have further smoothed it. And 2) I obviously cracked the cement casting. That's easy to fix though, just don't handle the mold until it's fully cured! ; )
I've also learned a few new tricks for printing these models that I'll add to my doc and repost.