Have some unexpected free time today and way I went measuring and seeing for alternatives.
And there are some, not sure if I'll went to do them, but let's share...
It feasible to arrange the contacts in the bellows directly, but, doing that, at least with the PB-4 we'll loose the chance to turn the camera to portrait. Here, and to accommodate a full sized body at least 10-12mm era needed. So no change here, let alone the Kenko one. Or try to electrify a PK-11, not easy at all, but doable with lathe work or some 3D printing.
At the front much more freedom is available and any type of extension is feasible. Again some will need lathe or 3D work to be adapted, or go with the smallest available, a Kenko 12 or a Movo 10mm.
Maybe I'll try the Movo...