RP appears to utilize broadcast packets in most of it's phone and gateway stat gathering.
I will agree, limiting the RP system to one subnet is extremely restricting. Especially for certain verticals where remote salespeople could greatly benefit from a remote client. But the system was designed to fit a particular demographic that fits a pretty large percentage of SMB.
I have heard of people getting some things to work across a VPN, but spanning multiple subnets is currently a breaking factor with RP, not just DLinks implementation of it.