The reason that electrical stimulation is not used is because the nerves conduct electricity and adding more current through them could cause the wiring to get "crossed" during healing. An electric toothbrush wouldn't be giving off any electrical output or you'd electrocute yourself when you tried to brush. Unless you're using some sort of incredibly high powered toothbrush, but I think in that case, synkinesis would be the least of your worries.
But in answer to the original questions, I had some synkinesis before my 7-12, but only around my eye. I do use one of those sonic care toothbrushes and have for a few years, but I swear I've never used it in my eye.
Keep in mind that true Bell's Palsy is different than facial nerve injury. Bell's Palsy is usually caused by a virus that affects the facial nerve but does not cause any physical trauma, like having an AN removed from the facial nerve would. Electrical stimulation for true Bell's Palsy is actually very effective, because the nerve fibers themselves are intact and there isn't the concern for developing synkinesis from the nerves "rewiring" themselves during healing - they're already wired -but the signals aren't getting through. Once the virus is taken care of, the nerve signals are able to move along the way they are meant to.
Lori