I did one on a 722.3 today. I don't think you can push the cover in that far, it's at least 1/4 inch to the o-ring. And if you could, you would have to be using some device to hold the cover in under strong spring pressure while you miticulously removed and replaced the o-ring. If you had such a device, you could just as easily remove the cover and piston to reach the 0-ring.
That b-1 spring is strong, probably 50 lb to depress past the clip.
I would envision some type of lever, like the old Chevy cheap valve spring compressors. If you can rig a lever, say with 4-1 leverage you could compress the cover, pop the circlip, slide the cover and piston and springs out, and pop a new o-ring in. But where to anchor the lever is the question.
With the tranny out, i could fulcrum it on the oil line bolt, like the factory tool mounts, but with the tranny in the car, you would need to fulcrum it above the b1 cover I think. Think baling wire, big c-clamp or something along that line.

DG