The question I have, goodly sir, is how one pumps the brakes and observes a
bottle hanging from the rear axle at the same time.

Maybe a lot of clear hose and have the bottle hanging nearby?

Hardest thing about building a brake bleeder is finding the right-size cap!
You'd think something from a detergent bottle or whatever would fit, but
noooooooo. Maybe a European detergent bottle but alas! I have none. Other
than that it looks straightforward.