I would assume at the least, a targeted dispel against anything coming into the area (that would be affected by the hallowed spell -- you could have it only affect non-good, orcs, etc.) -- that would be a roll against every spell effect that enters. And I'd say that anyone that entered, then left, would earn fresh dispel checks.
The scary idea is that it could be a targeted dispel against every affected entity, every round. That, I'm not sure about -- nothing in the rules seems to support or deny it. Seems pretty damned powerful, though.
I'm also not sure if a hallowed dispel magic will attempt to automatically counterspell (via dispel check) any spell cast by an affected individual. Also very powerful.
Hmm, maybe you pick one of the options, just like a regular casting of dispel magic -- targeted, area, or counterspell. Area would be fairly useless, since targeted would seem to affect anyone entering the area of the hallow anyways. But a all-the-time-for-a-year counterspell would be handy.
