This feature is coming - it will be called the VFS backend and you'll wrap your other backend like cache does at the moment. I have a test version of the VFS backend at the moment.
What is your use case - it would be helpful for me to know if I'm going on the right direction.
I just want to run lsl as fast as possible, and ddos google as little as possible. All files will be rclone copy into the cache.
Although I will say that my lsl commands got about 50 times faster when I got a new pc and new 5 times faster internet. But still, if I can cache the data needed for an lsl, I should.