Yes, I'd thought about a floating idler too. Good. A bit concerned that if I did use one, I'd want a minimum of 11/12T, so it'd have a diameter (plus required clearances) that would put the seat base way above its current height. So I'm keeping that option as a last resort, even though it'd be rewarding to have it working properly.If you used an Idler it would have to be floating or you'll end up with a compromised chainline as you run through the gears, as the chain run is rather short from the seat mount to the cassette.
So I'll cross out getting the base of the seat frame modified.Looking forward to see how you resolve this.
The alloy the seat frame is made of doesn't weld easily as it's tempered. Ask ICE.
In reality, I had not thought that an option - the ICE seat frames are incredibly light, and I don't recall ever seeing a post / report of the seat frame causing problems. So best leave as is (especially if a welder tells you they're not easy to weld.....), and modify the supports around it.
The lower chainline may not be so difficult a problem to solve - currently the steerer arm is straight, and very close to the pivot point; but a longer steerer neck, or a handlebar with a 30, 50 or 60mm rise, or an amount of rotation could all mean the return chain line clears the bar.
Frustratingly, I'll not be able to try out any ideas until the weekend.