You'll eventually run into a Mach Number limit, that's why the 20000 foot limit was put there in the first place, as a simplified means of containing the speed within the Mach Number limit.
Some manufacturers do provide the Mach Number limit, but most simply avoid it by stating a maximum altitude for Flap operation where the Mach Number is below the limit.
What on earth do you want to operate the flaps above 20000 feet for anyway?
Regards,
Old Smokey