Old Man Mountain makes such a rack.. I have had touring riders, successfully, complete tours across the USA using those..

they have you substitute the 10cm front QR skewer, with a rear 13.5cm for its added length..

the QR when tight is in tension , not much in shear.. the weight also using the shoulder of the lawyer's lips in the fork tip.


but As Typical , in this county has a lot of MTB riders. rural and mountainous..

they damage rims, and get new wheels , and the hubs and the QR skewers are piling up,

so you can get an extra rear skewer used, for cheap, and have it as a backup..


but I see you are talking porteur type rack.


....