BtS - we are probably into semantics here, but you are correct - the driver's handbook does not prohibit flight above 20k with flaps extended.
I suggest the intention of the restriction is self-explanatory? However, I do not expect anything dramatic is going to happen at 20001' in your scenario that would not happen a 19999', so I say "if you have to...................you have to". You would certainly get reasonable notice that things are not going too well, I think, as you climbed.
I can, like you, think of very few scenarios where you might want to do it.