That's the frustrating part. Despite being able to balance UDP connections, you need to use an http check. There's not even a tcp option.
We've resorted to running nginx on the servers and if we decide to production it, maybe we'll build something into monit to kill nginx if radius dies. Hacky.
Essentially it's just checking if the server's up in our case. Which is better than the round-robin stuff we have tried previously. Actually, previously we used Amazon's route 53 to route the traffic but this also needed a tcp / http check.
They claim they're not blocking outbound traffic other than the obvious (smtp etc.) but I don't believe that currently.
If radius works when we hit the server without the load-balancer and then mysteriously stops sending out, it would appear it is being blocked.