How to change valve seals without disassembling the cylinder head …

There are several points that must be successful when pressurizing the cylinder while changing the valve seal, so you have a good point there «DFIGPZ» When disassembling the cylinder head, you have both opportunities to check more details and make improvements

Not sure if you saw the edit to my last post ref pressurising multiple cylinders to prevent rotation - ultimately a full strip would allow re-seating of the valves and inspection of the chain idlers but it is " do-able"

Yes, I read your post and it is possible to run pressure on two cylinders at same time … You also don't have to dismantle the exhaust and carburetors.

I lock the rear wheel by stepping forcefully on the rear brake pedal, then take a piece of tie wire and tie the pedal down (usually to the frame) in that position.
For tie wire I use #12 solid copper insulated electrical wire.

Seen my dad do the "rope" trick on car engines. Never failed.

H.