Well, I use an Apple Watch when going for a run. It works pretty well to start/stop/shuffle/forward etc. the music -even while running.
However, due to JRiver's lack of "interest" in anything Apple, I very much doubt anything with regard to the Apple Watch is on the horizon. From my point of view JRemote and MC would first need to be better integrated into macOS/iOS/iPadOS before even considering spending effort on watchOS integration.