I ended up with an old straight key that has a curious feature: below the main arm that you would tap on, there's an extra metal flange that can sort of loosely pinch an extra sliding lever underneath, basically like a knife switch tipped over:
When moved into contact, it simply shorts the two terminals of the key. So I guess I could use it when trying to heat up an SWR meter or something, but otherwise it seems sort of useless, something you'd accidentally bump on at an inconvenient time.
Is my key wired up right? Seems more reasonable that this would be handy to disable a key that wasn't in use, but it sure doesn't seem like the electrical layout supports that. It seems intentionally designed to do the opposite, i.e. locking the key "down" instead of "up".
Would a telegraph or radio operator ever really need the ability to leave a "daaaaaaaaaaaaaaaaaaah" signal going indefinitely? Did this have a more useful purpose historically, like in some sort of common multiple-key configuration?