Though this space is "reserved for future addressing modes", I see no reason
why it is "bogus", especially when routers perfectly accept them.
 
I'd say their behaviour is undefined--do routers just use them like unicast addresses?

Reserved for future use is still the status according to IANA:
https://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xhtml


Hardcoding anything that is not loopback is bad idea, IMHO.
 
There are lists and documents about special-purpose IPv4 addresses. In fact, the IANA keeps a list of them:
https://www.iana.org/assignments/iana-ipv4-special-registry/iana-ipv4-special-registry.xhtml

I'd say BIRD should generally follow RFCs and object to using addresses contrary to their standard meaning.  Calling 240.0.0.0/4 "bogus" is too strong. "Undefined" would be a better, and probably as a (strong) warning.

Cheers,



--
     Jonathan