One company makes supertweeters which go up to 90 kHz – see the review here:
http://www.the-ear.net/review-hardware/ ... ertweetersWhilst the highest frequencies on CD are limited by technical considerations, other digital formats such as SACD could go higher, though practical considerations limit the top end.
https://en.wikipedia.org/wiki/Super_Audio_CDEven portable recorders can now sample at 96 kHz, which means a top frequency of 48 kHz. Theoretically analogue systems such as vinyl can go higher than CD.
I doubt that supertweeters would be useful in a Hauptwerk system, as the sampling frequency for most sample sets is 48 kHz. I would expect more benefit to come from additional speaker channels – to minimise intermodulation distortion and keep “pipes” as independent as possible.