Intruder - I don't think you quite have the handle on gliding. The flap system on a glider merely generates multiple polar curves; one for each flap setting. The speed for max. L/D increases as the flap angle reduces. For an airliner, this would mean that when cruising at high weights with high speeds, a certain amount if reflex flap could be used to obtain an optimum deck angle, thus reducing profile drag. At lower weights, lower speeds would be used and the flap setting would move towards zero from a negative setting, again optimising deck angle with respect to the relative airflow.
A glider's speed is also not binary as you appear to suggest. It's a little more subtle than that. It varies from minimum sink to best L/D to warp factor snot depending on the air it's travelling through and what the pilot expects next. If gliding solutions were applied to airliners we might actually save an appreciable amount of fuel, a number that might even impress the bean counters.
The reason we might not have reflex flaps were discussed by our mad scientist and he said it might result in the failure case being an expense we couldn't afford. But there again, we might reconsider this when we really have to start saving fuel