Continuing with Chas McDevitt's Skiffle book, I was surprised to learn—although I probably shouldn't have been—that Peter Hearn, who constructed a mini folding washboard for player and singer Beryl Bryden, made an electric washboard. I'd never heard of such a thing! The Bonzo Dog Band recorded a song including a solo on an electric trouser press, but of course that wasn't using electricity to alter or amplify the sound.
Checking the Internet I found a company in Spain, Traginer, claiming to make the first and original electric washboard: as they were only established in 2007 it looks like they were beaten to it by a few decades.
There's probably a general rule that if you can play it, a musician somewhere will have tried sticking a plug on it.
Presumably the washboard played on this occasion was not an electric model:
"One of Beryl's more obscure claims to fame is that she was probably the only person ever to have been recorded and televised playing the washboard under water."