David Cook wrote:
Anyone ever seen any of the garage-door-opener makers that
provide terminals so that one could attach an external indicator
light that would then light up any time the garage door was
open?

I put a set of magnetic contacts on my garage door and wired it to my
alarm system. When the garage door is open the alarm panel tells me so.

You could easily mount a pair of magnetic reed switches either on the
door or on the opener track.