"Any ideas?"
A couple, but unsure they'll work or they're practical.
If the 3750s support EEM, you might have a script that monitors the stack.
If you have ports to burn, you might define a VLAN per switch with two ports, then loop them. If VLAN unreachable, that switch is down. (Not very elegant.)
You might also check to see whether the stack logs anything with loss of a member switch. Something else you might monitor with an on-board EEM script or perhaps on a syslog server.
