Delegating military units to enforce the will of a civilian court is another. Posse Comitatus was originally brought in to end the use of federal troops to police state elections, so it seems unlikely that it could somehow be suspended to allow federal troops to oversee elections.
Caveat: I'm sure there are FReepers who know more about this than me.
Hmm: if SCOTUS ordered a re-election by paper ballot in (say) Pennyslvania - and the state refused - then that would seem to be a clear case for the Insurrection act?
If memory serves, Marines and Coasties are not under PC jurisdiction.