Neat idea for a thread!
Triathletes were using Scott bars in the 80's which were commercially available before Lemond debuted them in the Tour Prologue.
The first time I ever saw any version of the modern time-trial position was Pete Penseyres RAAM bike in 1986. It was more like a table-top apparatus affixed to standard bars than a tri-bend:



But that was just the first time I'd ever seen anything like it; there could have been something that preceded. I actually ran into Pete and Lon Haldeman on the road to Palm Springs once. He had those bars on the front of their tandem and they were changing a flat. They were pretty cool for Ultracycling legends, and I rode 202 miles that day.

Edit: Wow! Turns out Pete's record average MPH from 1986 remains unbroken to this day. (Clicky.)