After searching to see if the Volumio team had plans to incorporate Chromecast the short answer was that it wasn’t going to happen any time soon, with or without Google support.
I have a few old Chromecast audio discs which can output SPDIF or analogue audio. Since Volumio now has a USB PnP Audio device input I looked for a SPDIF to USB adaptor. One solution I found is a Cubilux USB A SPDIF Input Adapter which can be yours for £21.90 from Amazon. Of course there may be others that are cheaper but so far I am very happy that I can now stream from any Chromecast-enabled app directly to Volumio, shut down my iPad or phone, and the music doesn’t stop.
Cubilux make a range of adaptors, to output audio from a pc, to record to pc, or do both. This one works: ASIN B0BQQLFQ59
I hope this is of interest to those keen to use Chromecast in simple Volumio set-ups where SPDIF is otherwise not an option.