Emerging from a correspondence in one of Rick Walker's tribes, it transpires that Christopher Cohn spoke to Terry Riley about this matter and Riley confirms it was one of Schaeffer's engineers from the Musique Concrete lab, but he couldn't recall the name and was sorry he couldn't recall it.
So, yes it was a Euro-tape-sound-plasticity-familiar engineer who fixed up Riley so it most likely was someone with experience of the machines that were custom built to that purpose but realised with professional commodity gear. So I'll continue with the quest via their records. It has to come down to a handful of people, and I cherish the hope that we will be able to name him, and perhaps even that he was a composer/performer in tape himself.
And to reiterate, this is in no way detracting from Riley's status as a performer, composer and innovator. He was working to make a longer version of something he had developed with the Oliveros group in California, and was using already. The Time Lag Accumulator name came later, and was probably generalised from the name of the exhibit he put together then.
Thanks for everyone's ongoing interest in this matter