This has always been an issue that concerned me. The code really just applies to the installation as presented to the inspector, not what happens when they hear you slam the door on your truck and drive away.
I actually wrote a proposal to require wire to be sized to the largest fuse that would fit in a disconnect, just to see what they said. The answer is basically, you go on what is there, not what an unqualified person might do later.
The example was 400a switch gear with a set of 225a fuses in it and a 4/0 feeder. There were well over a dozen empty breaker slots in the load side panels and we were turning this over to state "maintenance men".
