I've found a 3D-printable renaissance rackett at https://www.thingiverse.com/thing:4603388, with no indication of where the designer got his information. The remainder of this note focuses on the renaissance variant of the instrument, rather than the baroque one.
A view from behind and slightly below looks like this:
A cross-section confirms that those holes penetrate into the final bore:
This continues the theme seen at for example
from https://www.willandbeki.org/construction.html, which the author describes as
The hole arrangement on the rear of the instrument - some are blind and some are functional. This seems to have been done as as sort of riddle to confuse the novice player or the uninitiated, or else it was just done as a playful visual decoration. If you look carefully at the Praetorius drawings you can see similar hole patterns depicted.
However in this case it's unclear what orientation is intended by "rear", I'm a little dubious about whether any of those penetrate into the final bore since a single small hole would surely make the instrument difficult to play while two holes spaced an inch or so apart would destroy the tuning.
But perhaps it is intended that flow through the bore should be limited, which would imply that the slots in the pirouette are intended to allow air to escape from the sides of the reed and omitting them will change the character of the instrument. This is as opposed to for example a pibgorn Pibgorn reed making where the reed is fully enclosed.
On a practical note, plastic recorders and flutes have been accepted- at least for teaching- for many years, and I see no reason why a rackett printed with a high infill should not work out well. However if I were doing it I would have separate body, cap and base components, and I would even be tempted to make the cap and base out of multiple parts to make sure that the bore was smooth, and possibly lacquer inside and out for a final finish.
Finally, my apologies for answering my own question: on the variant of social media on which I cut my teeth 35 or so years ago such behaviour was frowned upon.