Interesting I just fixed the opposite problem in my SP12 SolMan. I could assign the template from tech mon however when navigating to System Mon in the TechMon workcenter the query fails to return any values for web dispatchers.
My solution was to add SAP Ehp 2 Netweaver 7.0 (aka 7.02) for a product version which allowed me to check the diag relevant box as soon as I hit save the query in System Mon pulled the systems up without issues.
Also worth noting I have 7.41 web dispatchers in place, might install 7.42 if it allows me to check the diag relevant box and not have to come up with a work around to monitor a SAP product with a SAP product. 