Well, I don't want to test it and wake up the wife, but Airfoil now recognizes JRiver as the source and has successfully connected with my AirPort Express as the "speaker." WHOOOO-WHOOOO!!
If this works, then my living room just got a LOT cleaner and most of the gear needed for audio reproduction (Battery-backup power supply, RAID hard-drive array, and old, ugly white MacBook) just migrated to a better location. The only gear still in the living room will be the AirPort Express, Oppo disc player, AVR, and speakers - a MUCH cleaner look.
The next question (assuming that wireless streaming to the AirPort Express via Airfoil works as advertised) is whether JRemote on my iPad will still be able to "see" the JRiver Media Player on the iMac. I'd assume that since there is good wireless signal strength, since all devices are connected to the same wireless network, and since it worked fine before, that the chances are good. Of course, nothing is a sure thing...
It's taken more than a full day and LOTS of help from youse guys to make this happen. Ever in Baton Rouge? I owe you an "adult beverage" of choice! Thanks yet again - Boomzilla