Nimbostratus
Cirrostratus
That's a novel approach. I think you'd actually want to track open sessions--not connections. A single user could easily have many TCP connections open to an SSL VPN.
I haven't tried this before, but I wonder if you could use an SNMP-based external GTM monitor to weight the two pool members based on an SNMP query for the number of active user sessions each VPN server holds. Does the Juniper SA provide session counts via SNMP? The default SNMP monitor would just mark the pool member down if it doesn't respond to the SNMP request. You'd need to use a custom script to poll the members for active sessions and then adjust the member priority using bigpipe, tmsh or iControl.
Aaron