irule that would relay a DHCP request to a server on a different subnet?

Perhaps I am making a bad assumption, but you could configure that on your Cisco equipment (assuming you utilize Cisco), until the BIG-IP is given that ability.

 

 

It is called a Cisco DHCP Relay Agent.

 

 

http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfdhcp.html