the belay loop...or most any other material for that matter, only cares about a peak load. not so much about residuals.
typically, a residual load can be less damaging than cyclical loading, even if those cycles are a lower magnitude.
also, the belay loop of a harness is nothing more than a beefed mini-sling. are you concerned about slings you use as anchors in top-ropes or hanging belays?