Bill, would it be possible for you to add the -d option to relayd on your relay unit and run a few DHCP requests to see what it should look like?
I don't know how to add arbitrary options to daemons in OpenWRT using configs, so one way to do this would be to SSH to the WiFi client's address, do ps | grep [r]elayd, note that line, then stop relayd and run it manually by adding the -d option to the noted arguments.
that way it will show all requests and responses and hopefully that will be different from mine. I checked the sourcecode and it seems to be validating quite a few things in DHCP packets before forwarding.