Interesting. That dummy pool member idea was what I was going to suggest along with priority group activation. But thinking about it again, I'm not sure it would work easily as the higher priority pool members would still be selected even with the lower priority group was used.
Do you have the actual servers defined in real pools which you select from the iRule? If so, could you use the active_members iRule command to check if the pool has enough members to use it? This would be in combination with a standard monitor configured on the actual pools.
Else, I would think you could use a tmsh script to get the data you're looking for from the command line. Unfortunately, I haven't played around with the commands enough to suggest anything meaningful. You could try posting the question in the tmsh forum: http://devcentral.f5.com/Default.aspx?tabid=53&view=topics&forumid=70
Aaron