A followup: I recently did an older 1-1/4 angle cock with conventional tapered brass plug key.

After lapping, I lubricated with Lubriplate 130-A. This is a calcium sulfonate grease, NLGI between 2 and 3, and has a zinc additive. The manufacturer recommended it as having excellent compatibility with brass and bronze, and having excellent tackiness and anti-washout properties. I mixed in a small amount of dry graphite. So far I'm pleased with the results. I guess we'll see how it holds up after a year.

I've been wondering while following this thread: Is there any concern with with stray lubricants gumming up brake valves?

If the lubricant is applied excessively, I can see it migrating into the ports when the key is rotated. But the same properties which make it desirable as a plug key lubricant, the tackiness, adhesion, and wash-down resistance, would also make it unlikely that the small amount of lubricant which actually remains in between the two metal surfaces would migrate downstream.

Go to a plumkbing supply and buy a small tin of silicon stopcock grease. It should work.

That's what the Molykote grease I mentioned earlier is. A silicone grease with NLGI between 3 and 4.

I'm going to do a "bake-off" next and try lubricating the same 1/2" cutout/drain cock with various greases to see what works the best.