In theory, yes, we could support additional amino acids in Foldit.

However, Foldit does not do any atomic orbital calculations (our modeling engine uses simple ball-and-stick molecular mechanics). I think the only difference between SeMET and MET would be a slightly larger atomic radius at the Se position (?), which is unlikely to have a serious effect on Foldit protein models.

We can also model metalloproteins, insofar as we can include a metal ion with the appropriate charge and atomic radius. But, without orbital calculations, we typically have to define the metal coordination geometry by hand.