Functional bikes. Not Porn not Anti

I hadn't seen those extended nut things, might give them a go, figure the extra bar should be fairly easy to remove...

If there's a rack supporting the bag (and I'm sure there is) I can't see that option being a problem.

Exactly this.

However a cheap decaleur may not be a tight fit unlike those super-expensive one from Compass (Grand Bois), which have no movement when slotted in.

As long the bag support the weight, the decaleur simply held it in place, very much like the rack stay on the seat stay, the seat stay mount are far from strong but enough to hold the rack in place while the weight is distributed on the drop out.

FWIW, I find my Grand Bois system work brilliantly, even over 10,000km I never have to make any modification or change, while the other modern system are heavier, and more prone to vibration (even with the Klick-fix, the bag bounce notoriously, and excessive vibration pull the bag downward regardless of the make).