OSPF type broadcast over GRE for NRHP

I am trying to use BIRD to run OSPF inside a Phase 3 DMVPN using OpenNHRP. The problem I'm seeing is that it appears to be impossible to force type broadcast on a GRE interface. When I try, the message "Cannot use interface gre1 as broadcast, forcing nbma" is logged. This may normally make sense, but the OpenNHRP daemon captures multicast traffic on it's interface and forwards it appropriately. A configuration parameter to bypass this protection and allow broadcast on a GRE interface would be very helpful.