Currently, yes, that would be one way to do it (although your withBroadcast arguments seem questionable). You would have to have multiple network interfaces or multiple addresses on an interface. Another way to do it would be to implement your own transport based on routing between the networks that you wished to bridge. It looks like Terry may have fixed a bug concerning reusing the IP address and port recently, so perhaps you can pull and see if that will help you do what you are trying, https://github.com/infiniteautomation/BACnet4J/commit/9cb0864ef25be802b0b2e84ebd7013018bfafac3
Broadcast addresses are all ones in the address bits on a subnet usually, unless there is some unique routing going on there. I.E. 192.168.255.255/16 or 192.168.1.255/24