The "flaps" will have a large pitch authority on a flying wing... you'd need to mix to the elevons to essentially have a "crow" setup. % relationship depending on sweep angle of the TE and relative areas of the surfaces.
This does give the effect you are after, adding significant drag to allow a steeper approach without building speed. It also makes the plane less likely to drop a wingtip in a stall.
I'd do a variation on the 4-servo sailplane wing mix, allowing flaps to follow the elevons, elevons oppose flaps.
When we did wings using separate elevator vs ailerons it was appx 60% aileron, 40% elevator in the center.
Experiment with LOTS of altitude... When you put flaps down the nose will go down.