The Autocourse history of McLaren. I checked it out on a few web sources, and sure enough, John Barnard had worked for a few other teams before. No other site I (later) checked had Rebaque team listed...that doesn't mean it's not out there somewhere.
What I should have asked was "Where was John Barnard technical director for the first time?"
Doh!
By the looks of things, JEC gets the question right, because the Lola team was in fact making F1 cars (for Honda, and Barnard was working in the Formula Vee section). But for me to ask the question about which team: Someone working for a team that entered cars made by another manufacturer, by way of someone else designing it...not a fair question at all.
But that's like saying Lord Hesketh worked for Ford Motor Company because his F1 car had a Ford-Cosworth DFV bolted to it. Next time, I'll wait (and think) a little longer before posting another question!