It looks like you MIGHT be able to cobble something together with the use of this project to provide the Chromecast Audio interface: [RELEASE] cast-web 1.0.0 - Chromecast Device Handler & SmartApps . Possibly with Webcore in the middle… it would depend on whether that Chromecast handler can “see” and use the virtual SmartThings buttons that are created for the Pico buttons. I would guess that it might be kind of sluggish for continuous-press functions like volume changes.
One note of caution: that Chromecast handler has an architecture similar to that of LutronPro v1 (and LutronPi v2): a SmartApp and its device handlers talking to a node.js program out on a LAN device (like a Raspberry Pi or other computer). As noted above, SmartThings only seems to allow one internal device per LAN device port, so you’d have to host the Chromecast handler’s node.js program on a LAN device other than the one hosting the LutronPro/LutronPi node.js program.