There is an electronically activated wireless dropper on the market, so with a speed sensor and a little imagination I'm sure you could rig one up to do this. It would suck to have it drop all at once if you weren't ready, and you'd have to raise your weight off the saddle to make it go back up.

Really,, having a cable operated post with a little lever next to your thumb is super convenient, if you're into that sort of thing.