Any question answered...

Since most hub manufacturers use proprietary freehubs, you'd have to hope that a manufacturer who makes your 150mm hub also makes a 135mm SS hub with the same freehub interface, at which point you might as well just buy the 135mm hub :-) That would certainly save the trouble of making a 135×10 axle to fit a 150×12 hub.

Yes, I'd Hope (ba dub tish)... but they're not cheap and I was hoping for nice wide wheelbuild based on a niche (low demand, low competiton on ebay) hub.

Plan would be a 142x12 rather than 135x10(5mm qr).

The easy option is a DMR Revolver disc SS hub, but wide-foot cassette sprockets are cheaper to keep as options and are less risky to change than a standard 24x1.37 freewheel.

Shimano DXR FH-MX71 body http://www.sjscycles.co.uk/shimano-dxr-fh-mx71-complete-freehub-body-3dw-9803-prod29219/

has a similar interface to 3CN 9808 (M970/M975/DA...http://www.sjscycles.co.uk/shimano-xtr-fh-m970-cassette-titanium-freehub-body-3cn-9808-prod29201/

Not saint or zee though, so no wide hub shells.

And DXR isn't a thruaxle as far as I know.