Could I put an slx lever on an old style caliper (retaining pin on one end rather than central ) and expect it to work?

I run M988 XTR levers on both Saint M800 and M810 calipers with no issues

didn’t the diameter of the hose change at some point?

By the time you mess with cabling and olives it might be as cheap to just add a M685 caliper as well ??? (Assuming you already have the levers)

M7000, M8000, M9000, M820, M980, M985 run a different hose, hence the bigger insert & olive.
I ran a M666 lever with M665 caliper with the small insert – it end up leaking from the reservoir.
The point: use the new insert & olive and you should be fine with newer style brakes

The point: use the new insert & olive and you should be fine with newer style brakes

True but then an SLX caliper is about the same price as a new hose kit… so if it ends up changing hose just sticking on a SLX caliper could be cheaper and not worry about compatibility…

The M685 caliper is (so far as I know) really the M785 with a different label and a split pin supplied but even the thread is there to put the M785 bolt!