My view is that you should try not to extend flaps until you are within 10kts of the block speed. ie if clean speed is 210, then don’t select flap 1 until you are below 220. This is simply being kind to your flaps! I don’t think it matters whether or not you are in the descent.
If you need more RoD then use the speedbrake. If you still need more then you should probably be telling ATC that you need more track miles. If you are doing everything that you reasonably can to get down and it is still looking tight, then tell ATC rather than fly in a non-standard config and get yourself boxed in.
Re the 20,000ft limitation, this is from the Boeing Airliner magazine:
"Several operators have asked Boeing why the Airplane Flight Manual has a limitation restricting the use of flaps above 20,000 feet. The reason for the limitation is simple; Boeing does not demonstrate or test (and therefore does not certify) airplanes for operations with flaps extended above 20,000 feet.
There is no Boeing procedure that requires the use of flaps above 20,000 feet. Since flaps are intended to be used during the takeoff and approach/land phases of flight, and since Boeing is not aware of any airports where operation would require the use of flaps above 20,000 feet, there is no need to certify the airplane in this configuration."