I know this is a very old thread, but I have this issue as well. I'd really like to see gapless support with the Chromecast Audio and JRiver MC. This is not inconsequential for someone who currently has 17 active Chromecast devices (14 active CCA, 2 CC with Google TV, 1 CC Ultra) in one home.
Foobar2000 is not the only program that has achieved gapless support with Google devices. I'm currently doing a free trial of Roon. It has this CC gapless support as well. Not gapless over DLNA, but DLNA isn't a must. What's important is that the audio get there uninterrupted. And I have to say Roon has done it beautifully. Roon has a lot of shortcomings compared to JRiver MC relying on cloud service for search, very minimal library editing capabilities, useless integration with Home Assistant (this works with DLNA for JRiver, cough cough) in no particular order - and of course a so-called lifetime license is a hefty $829, probably more than I spent on all my JRiver master licenses. I might be willing to deal with a few of these shortcomings, but taking them altogether, I find myself wishing JRiver would add the support for gapless on Chromecast somehow, or perhaps someone else could through a rendering plug-in, though I'm not sure the plug-in interface supports doing something like foo_upnp.