So, one of my coaches went to an IBJJF rules certification seminar. He said that you could do a knee-lock two ways from the lockdown/electric chair. One is with the locked-down leg itself, torquing their knee inward - or if you're below brown, an actual straight knee lock (more likely from top). The other is you stretch them out and create a "violin" like lock on their leg that's on your shoulder. It's very blurry and will be referee dependent, although most of them know now what the electric chair submission is and the sweep. (Thanks to M3 and Eddie.)
edit: should be said that most people familiar with the EC don't get "trapped" in the knee locks. It's a panic thing. More like a "check" move than a "checkmate." They can escape it, they just don't see it.