priority group activation

This is pretty much what I'm running in an UDP scenario on one of my customers that requested a similar feature.
I build an "external monitor" that triggers a connection to the other pool members (passed as script arguments), and waits for a response. 
- if returned data is >0 there's an "exit" condition so monitor fails 
- if returned data is 0, it checks the next pool member 
- if all checked PM's return 0 data, it echoes a "KO" message in ouput so monitor turns green and node is eligible for traffic

This will allow you to "force" LB reselection 

This ext-mon object is then configured only on lower priority pool members, additionally to the health monitor of the service. Works flawlessly, just be careful with probe schedule since if you open lots of sockets the script might as well intercept some response data from another connection, possibly resulting in false negatives.
[EDIT] Since you're checking external objects, one last thing I'd recommend is to configure alies address and alias port in the monitor object, so that a single probe updates the status of all your pool members (otherwise, it uses pool members' IP:port as different sockets, which builds up to multiple unnecessary probes)