Thank you swiv3d, an interesting solution indeed (really nice of "Don W" to share this)! I just tested it.
I have 2 problems with this :
1) It doesn't work remotely. If I set this up on my server PC, the Youtube launching app will work in JRiver in theater mode on the Server PC. It won't however launch in my JRriver on the client HTPC.
2) It's not a "clean" solution: before the launch of and after quitting Chrome, there's a quick flash of desktop & other instances of Chrome.
I fear that the absence of other answers means that there's no Youtube.com/tv in MC24's Theater mode. I hope that the JRiver dev team gets that Youtube/tv is an ESSENTIAL & HIGHLY EXPECTED feature (see the numerous messages about it). This mode is really neat and SUCH a natural fit for JRiver! There's no question that this would be the most impacting new feature that we've seen for a long time!