I've seen the shut-off valve, but has anybody capped off the heater core hose outlets at the engine, to basically remove the circuit completely?

What do you use for caps?

Capping them off wouldn't be any different as far as cooling or circulation as having a closed shut-off valve, right?

no idea...I made my own 'in line' shut off valve as nobody knew what I was talking about, at Oreillys, pep Boys NAPA etc...got parts from Home depot...cu the heater rubber line up high before the carb ,so its easy to get to...I turn it twice a year...winter use, summer use....no idea about caps...just think caps would be lots more permanent.
Mine didnt leak fluid to cabin ,just wouldnt shut off hot air flow all the way...and in Texas summer heat 90-105F ,its no fun with hot air on your feet.

I had mine capped for a while until I replaced the core. I'm a fan of capping instead of bypassing. It's better to have the coolant pass through the head.

I think they are simply NPT fittings but I am not absolutely sure. What I would do is unscrew one of them and see if you can thread into a female thread of the equivalent size. If so, buy a simple plug from the hardware store.

My heater core went out a few years ago. I dissconected the lines from the heater core, and looped them with a piece of 3/4 copper pipe and some hose clamps. no problems since!

I think I bought a set of various size caps at the auto parts store. They are rubber caps that you attach with hose clamps