Thank you for gracing me with a respectful answer. I've come to the conclusion that there probably is no practical, electrical option (reed switch, for example) but 'mind hacks' are probably best.
Just saying "learn to unplug" is not exactly helpful. Im amazed how people think they are contributing to the world with such comments.
Anyway, thanks for all your helpful suggestions. Notes-to-self will be liberally plastered on my gear, and nasty swinging objects hanging from all the door posts in my house!
Ok, maybe not the second one...
To be honest, though, "just learn to unplug" isn't actually an unhelpful response - it's a purely practical and pragmatic one. The way you develop a habit is to repeatedly do an action. And, ultimately, the best way to do that is just to force yourself to do it. To determine to do it and to keep mentally reminding yourself to do it... Which is what "just learn to unplug" boils down to. It might not be expressed elegantly and at length. But it is valid advice. It is what you need to do, when it comes down to it.
Sure you might find some things that kinda help you remember... LEDs. Post-Its, tennis balls... but there's a problem with them which means you can't rely on them, they can only enhance the effectiveness of will power.
They quickly fade into the background.
So, put a Post-It next to the amp off switch. Great. But by the tenth time it'll probably just be a part of the front of the amp, as far as your subconscious mind is concerned. You won't look at it, you won't read it. In fact, you probably won't even notice it. If you haven't developed the habit by that point then that hack has gone and you're back to square 1. Leaving the bass plugged in...
Determination and repetition... that's the key. And, to be honest, of all the good habits you might want to develop in life, this is hardly the hardest!

