Are the stays on an old PX-10 frame (531) sturdy enough for disc brakes? I ask this, because this frame has nice, wide stays for wide rims and tires for use as a bad-weather bike. I would consider having disc mounts brazed on if I knew the stays could withstand the forces.
Answer: Its not worth doing, from a financial stand point.
Adding disk brakes to a frame is expensive, risky, and ruins any vintage value the frame had. The stays are not engineered or designed for the new sideways loading.
Also, you talk about stays, but braking on the rear wheel is nowhere near as good as braking on the front.
If you're dead-keen on fitting disks to this frame, then a better solution is to buy a new front fork with disk mounts, a new front wheel or hub with disk mounts, a front caliper and rotor, new cable/hose, and a new brake lever that suits. Keep the old fork and parts too, that way you can revert to original spec for no cost.
If you want a disk brake bike for bad weather, buy a second-hand MTB and fit mudguards, leaving your loverly vintage bike for rides.