I had not thought about a use-case like this before, and had a look at loopback in ALSA.
If you create a loopback device, then this device is selectable as a soundcard in the Audio Entrypoint. If the other end of the loopback can be selected as the output for Squeezelite then this might work. The test I did (on DietPi) is playing a wav file to the loopback device and then capturing from it with the Entrypoint and that worked.
Such a loopback setup might also work with Raspotity to create a Spotify connect client.
I would love to hear if this works, gives a new set of possibilities!
The guide I used to setup the loopback device is this one:
https://sysplay.in/blog/linux/2019/06/playing-with-alsa-loopback-devices/