John,

Ah, we're talking at cross purposes;
....
I think I've seen them where, although the mobo connector is two row

My bad, I can't remember having ever seen them in a single row pins.
Regardless, I should have directly mentioned a 2x5 header/connector instead
of starting with just "10 pins".

They seem, these days (or three years or so when I "retired"), to be
becoming less common anyway: two-chip LEDs yes, but with three legs.

Yup, still have a few of those. Athough you could keep toggeling the
polarity on the two-pin ones to mix colors the three-pin ones where way
easier - fire and forget. Than again, with almost every second chip being a
programmable controller of some kind it doesn't matter that much anymore.

I can't remember which parameters: either forward voltage against current
at a fixed temperature, or forward voltage at a fixed current against
temperature;

I must confess that I never went that deep into it. Have seen a few zener
curves on a 'scope ofcourse, but just for the "proof of the pudding", not
much else

Having the LEDs show the circuit was powered was indeed also useful!

True. But at some point I got enough of seeing them literally
/everywhere/ - with no possibility to switch them off without de-powering
the whole device. :-\

Regards,
Rudy Wieser