Firstly, the manual says to lock them for at least 50km per month (speaking under correction here), to get lubrication spread properly. It gives no limitations in terms of speed and duration.
Also, if you look carefully in the manual, it shows a fixed drive flange option, which is presumably fitted in some markets.
Deducing from these two facts, I seriously doubt you can do any harm, as long as you grease the front prop-shaft at the same intervals as the rear.
On proper overland trips I tend to lock my hubs the first time I hit dirt road, and leave them locked until going through the border post on my way home. Thus I have left them locked for a couple of thousand km at a time on a number of occasions.
I have yet to notice any issues resulting, and can't even see any negative effect on my fuel consumption. In fact my two best consumption figures have both been achieved with hubs locked.
Mag ons ons kenniskry met lekkerkry aanhoukry.