Dear Rafe,

Currently you cannot recover ancestral sequences from Datamonkey.org, although this is probably a good feature to have.
I'll put in on my feature request list.

If you run SLAC locally in HyPhy, you can choose Analyses->Results->Ancestral Sequences and choose 'lf' from the pop-up menu then select where to save the alignment with your ancestral sequences.

Cheers,
Sergei