Given the pitch limits of capped reed instruments, I wondered whether some means of expanding the range of such existed. Perhaps with some sort of embouchure mimicking device surrounding the reed, but within the cap. The question assumes that the bore is not changed from the original conical/cylindrical bore native to the instrument(for instance, the crumhorn/rauschpfife, respectively).
It's already been done. George Kelischek's rauschpfeiffen can be "overblown" into the second octave: https://www.susato.com/collections/rauschpfeiffen
Otherwise, no I don't believe it's possible to extend the range past nine notes without fundamentally altering the instrument. I've played some capped reeds with keys which extend the range slightly (a couple notes).
George's Kelhorn is a fine example of such alterations, and might prove to be the ultimate advancement of capped reed design: https://www.susato.com/collections/kelhorns