note: no runway analysis charts provided to cover this scenario
You're walking into a mine-field in No Man's Land, DON'T THINK ABOUT IT.
The required data could undoubtedly be obtained from Boeing, for a price, but would your operations justify the price? The only scenario which might make such a proposition worth-while would be if The Flaps 5 RTOW exceeded the Flaps 15 RTOW (which undoubtedly it would in many circumstances), and thus a lesser thrust might be required at a weight between the 2 limits with Flaps 5.
The GENERAL Boeing philosophy (and probably at Airbus also) is to provide reduced thrust data for the "NORMAL" day to day flap setting (in your case Flaps 15), but if increased weight is required by resorting to "unusual" configurations, e.g. Flaps 5, Packs OFF, Increased V2 speed schedule (Improved Climb), then the default is to go all the way to Full Takeoff Thrust (there are exceptions).
Regards,
Old Smokey