I was looking at the normal way that factorio generates resources today, and noticed that there's a built-in way to provide a piece of lua code for factorio to calculate richness and probability values for ores for an individual tile.
Now I started wondering, if it would be possible to change RSO, so that instead of generating ores using a custom function in control.lua, it would generate ores using the same custom function, but inside of autoplace-expression functions.
This would allow the map preview to work, and it might even increase performance a little bit, since iirc, generating new chunks was one of the things that was done multi-threaded in the base game.

oh, and yes I am aware that this would probably be a tremendous amount of work, I'm just wondering if it would be possible at all right now.

I'm not sure - I didn't go into autoplace expressions.
I'm aware of at least few things that RSO does that would be difficult in there like being aware of already generated ores and moving new field based on that.
If you can use arbitrary lua there then you would need to precalculate everything as it is right now and then feed the bit in autplace expression.
From what I looked most performance cost in RSO is checking what is on the map tile and placing the ore then. Actual cost of generating the patch is very small compared to map operations.