Thanks for the suggestions, but I think maybe I've not explained the problem well enough.
I can get WMC to pick up the remote control signals by playing around with the Media Key settings. However, I then can't control the volume of loopback. I think I have set-up loopback correctly: Windows thinks it is outputting to an unconnected on-board sound card, but JRiver then picks up this signal and outputs to my PCI soundcard. However, from what I understand, this means that the volume now needs to be controlled within JRiver and can't be controlled through other Windows applications.
I suppose an alternative approach would be to make JRiver aware of the system volume, so when that is changed outside of JRiver it affects the loopback volume. Is this possible, or is there no way to adjust the loopback volume when in another application outside of JRiver? If this is the case, loopback seems a great idea, but I'm not sure how practically useful it is.