@erik, are you forgetting about the Apogee Acoustics full range ribbon loudspeakers?
But let me back up. I asked if you meant to say "ribbon" in your sentence "this is not the first time a planar midrange has been envisioned". I did so because the "this" in the sentence I took to be referring to the speaker the OP is asking about, a loudspeaker with specifically a ribbon midrange, not a generic planar one.
In regards to planar midrange drivers, of course the Piega 511 is not the first speaker to employ them! My God, that goes back to the 1950's. QUAD, KLH, and Dayton-Wright ESL's, Magneplanars in the early-70's, the Infinity IRS and RS-1b, all the ESL's introduced in the 1980's and later, the ET LFT's, and many more.
So Erik, what do you mean by "a planar midrange"?