Yes the seatpost clamp idea will work, if you dare there are special minimalistic 1 bolt clamps from problemsolvers
. But they don't dare to state this kind of use with their product, so probably better not
Many BMX brands still have 28.6mm clamps, you can even have them with double bolts, so it is exactly as strong as a regular stem with double bolts. But this won't look very nice though.
For installing you would need some sort of press (long piece of thread with big rings and nuts on both sides will do) to adjust the headset play. Or fit a starnut temporarily and after you tighten the clamp you can take out the headset cap and the starnut.
There are even special headsets to take out the play. ACOR made them for instance; it's an ahead set but with a nut and threads in between so you can adjust the play without the need of a starnut or topcap. I've added a picture, you can just about make out the headset that I'm talking about.
All that said: Just get a fork with threads
everything is better when it is just installed the way it was supposed to, without adapters and stuff
My WANTED list:
- Lefty; preferably post 2013 version. - C'dale Headshok 80mm DLR - SuperV / Raven Swingarm
- Magura louise Raceline yellow - purple 3DV ano cranks - Wide handlebars (700mm) 25.4 clamp
- Seatpost and clamp for Manitou - White/Grey Tires (psychos) - Hutchinson Scorpion UST grey
- RS Judy Crown with 1 1/8" steerer (Thread or Ahead)