To anyone who may be interested i can confirm that the 29.8-28.6 tandem seatpost shim as sold by SJS works a treat if you ever want to put one of these crazy stems on a regular 1 1/8th" steerer. Regular top caps with 31.8-32mm outer diameter get enough purchase to pull the stem down, too.
A 25.4-28.6mm stem shim, split in half, works a treat at the bar end of the stem for drop bars with a 25.4mm clamping diameter and with some teasing a 26mm will fit in there, too. All depends on the combination of the tolerances of parts involved, with the latter, so YMMV.
Handy having the larger bore at the bar end of the stem as you can wiggle bars through it without scoring the bars to bits. Have tried an SR road champion randoneur bend and a Soma Highway One compact with success.
Happy crimbo!