Everything appeared to be fine after testing. I changed a few suggested settings, but nothing changed. MC played 1 song, then stopped. I noticed that the groups had vanished, so I looked in BubbleUPnP, and they're gone from there as well. So not sure what's different about Chromecast groups. I still see the devices themselves in MC and BubbleUPnP.
I opened up VLC and it played to the group, but it identifies it as a Chromecast group, so I think it's somehow actually casting, not seeing the DLNA device that I'm making with BubbleUPnP.
I also can play through MO 4Media, which casts and that sees the group without issues.
I restarted BubbleUPnP and the groups showed up again - so something is certainly amiss before it gets to MC as a DLNA device.
Has anyone tried using Chromecast groups like this? Another wish for MC to support casting directly (like whatever VLC hacked together).