Do you know how can I ask this feature will be implemented in future versions of JRiver MC?
JimH, the owner and CEO of JRiver, reads all these posts, as do the developers. So you have already asked for the feature.
But I don't think JRiver can provide a solution to what you want. It is a network level, and DLNA standard, issue.
What you can try, with no guarantee of sucess, is to set up a VPN from the remote location to your home, so that all three devices appear to be on the one subnet. That isn't as simple as it sounds, as you can't just set up a VPN from your Surface to your home, because that would leave your Panasonic NX-30 still connected on its local subnet, and so not visible. I doubt that the Panasonic NX-30 is capable of connecting to a VPN itself.
So you would need to set up a VPN from the router in your home to the router in your remote location, and have both the Surface and the Panasonic NX-30 use the DHCP server on your home router to get IP Addresses. All three devices
should then be on the same subnet, and DLNA
should work. But again, no guarantees. VPNs can be a little tricky in this sort of configuration. Settings your Panasonic NX-30 to use a DHCP server from your home router, only when the VPN is active, and elsewhere at other times, could be an issue. You might get away with using a Static IP Address that matches the range used in your home router, but networks being what they are, that may not result in the Panasonic NX-30 being seen as on the same subnet.
But using a VPN, or hardwiring from the Surface to the Panasonic NX-30 via USB, HDMI or something, or using a Bluetooth receiver connected to the Panasonic NX-30 via USB, HDMI or something, are your only options I think. Of course Bluetooth has audio quality limitations to consider.