So my front caliper appears to be leaking. Poor braking and loud squealing coupled with what looks like fluid behind one of the pads.

Could warranty it but really can’t be arsed to send the caliper back to madison. All the RS785 calipers appear to be out of stock everywhere. Can I use another Shimano caliper like an XT?

You can use any other Shimano caliper to replace it XT would be an equal exchange in terms of pistons etc.

Hose mount looks slightly different on the XT caliper?

I’ve used XTR M9020 callipers with my R785 levers. They work perfectly.

If you replaced it with an XT calliper you would need to replace the hose too. The 785 calliper has a straight connection, the XT a banjo connection.

Replace the entire hose or just the connection?

The connectors are crimped onto the hoses so it’s a new hose.

You can swap the position of the bleed nipple/hose input simply by unscrewing and swapping them over.

Is it definitely the caliper leaking and not just the hose connection? If caliper is ok just buy new hose or buy goodridge hoses. I got pig sick of leaking shimano brakes so splashed out on goodridge and the problem went away. Sadly the goodridge hoses do not address the huge amount of lever throw shimano have but otherwise correct several faults by using screw in great quality stainless steel fittings.

CRC have several models of Deore caliper where the hose connects directly to the caliper. You’d should just need a new barb and olive.