They did emergency drops into Oosterbeek. They were largely unsuccessful - supplies landing just beyond the perimetre - but they did try to do it.
And as far as road supplies go, if a column was interdicted up a road they were using for supply, are you saying that if there was a healthy diversion available, just 10kms to the flank, they wouldn't have simply re-routed the column to there?
So, for example - the pic below. Supply down main road interdicted at 1. How long would they p[ut up with that when, off map, there are possible re-routes to the new position marked 2?
In a good scenario design, ground transport supply entry point(s) should reflect routes from depots to the map edge secured prior to the start of the scenario, or the location of a depot itself.
The Germans in Bulge would usually have access to SEPs along the east edge of a map because roads coming from the north, south, and west of what became the salient boundaries were controlled by the Allies from the start.
The designer should also consider that not all roads on the east edge of the map were viable communication lines to German supplies. Some on the north and south ends of the FLOT might wind off map through allied territory before bending back to the east edge of the map.
Parachute and glider supported SEPs are different, because they are points on map where planes are scheduled to leave supplies.
If there were an ability to change them, it would be based on how effective communication was with rear commanders and how quickly the rear administrative engine could plan and implement a new drop point. Communications from isolated units is significantly less effective in World War II, and the paperwork generated to replan multiple carbon copies of typed documents and forms carried by messenger from point to point in the ETO administrative hierarchy.