From memory our semis in 2003 and 2004 had low crowds. I think it was just under 8,000 v Dundee on a Sunday at 3pm then 13,500 v Dunfermline with a 3pm kick off on a Saturday. So less than 20,000 v Hearts with the game live on TV doesn't seem all that bad for creating an atmosphere. The lower gate money from this is a bit of a bummer though.
I also think that most of the Jambos would more easily get to Hampden from Edinburgh for a 12.15 ko than our fans. 1.30pm at Tannadice or Dens on the Sunday might work better but the other semi ko time would have to be moved to avoid a clash for TV.