NightHawk nice enough to send me an EKAR lever and..........
No problem. Have to use all the rear sift guts including the funky thumb button from disc outward...and the 13s tooth bushing in the front. The 13s/EKAR shift lever blade is slightly different shape but the 13s shift lever blade would probably work fine in an otherwise 11s/12s body.
As I suspected, Campag makes some small changes but the basic design is the same as 11s/12s UltraShift.
So wanna a rim brake, 13s 1by rig?? It IS possible altho a lot of effort and $ for just one extra gear.
Ekar bits in a 10/11 speed body:
https://forums.thepaceline.net/showthread.php?t=290764