i wonder if there's a market for pre-made cables with a "loop" in the hot wire, for using clamp ammeters, or a cable where in addition to the choke ferrule, there's another "ferrule" - current transformer - on the hot wire with "test points", where those test points are to a transformer winding around the hot wire. That way you could just put those cables on things you want to know the power consumption of, rather than having to move a kill-a-watt or whatever.

I could make either, but they wouldn't be "certified", as i'd be either replacing plugs or cutting into the wire itself to add a pigtail.

You may be looking for "AC line splitter".

One example: https://www.harborfreight.com/15-amp-professional-ac-line-sp...

Edit: s/splicer/splitter, damn autocorrect.

Matthias Wandel measures power that way: https://www.youtube.com/watch?v=P47pjVyPP3w

Yup. They even typically come with separate 1x and 10x loops.

They're a common pack-in item with low- and high-end clamp meters.