A conversation with
@Mickeyspinn [oh crap, did I listen to this guy again?!] made me go to the shed and I came up with the following combination of a cheapo quill adapter, FSA carbon fork bung and a star nut.
It's mickey's idea, but not having a metalworking lathe, I figured out a way to use spare parts from the shed.
Easy hack with minimum drilling.
Drill out the nut to allow the quill bolt through. Hammer the nut into the adapter. [Photo is wrong - presumably, need to hammer back-to-front as it will have to resist pressure from the top...
Or you can hammer 4-5 of these into the adapter until they ain't moving down under the quill bolt load]
Drill out the FSA bung to allow the quill bolt (but not the head!) through.
Assemble. Tighten the quill bolt. (Fsa inner rests on the star nut inside the adapter body. Bolt rests on the FSA inner.
Add spacers.
Screw on the top cap to preload the EC30 headset bearings.
Two things I don't like about it.
1 is that these cheapo adapters have alloy wall thickness of 1.95mm (In contrast, a better quality adapter measures 3.75mm but won't accept the FSA bung without further serious bodging). Put a deep scratch on it and say hello to a stress riser with ~1.5mm wall thickness!
2 The slightest movement of the quill adapter will mess up your bearing preload.
Anything I've missed?
This is not my first choice for the threadless bodge, but I think it's workable. Usual caveats apply, don't blame me for gross misuse of bike parts if things go wrong.