Well, CIP pressure standards are available online
here, and they say 830 bar max average operating pressure, so just under 12,000 psi, same as other shotshells. My biggest questions about reloading them would be:
* how are you going to reload a rimfire case,
* why would you even want to try, considering the challenges with pressure testing and load development and the lack of wads, shot cups, or other components
and
* since commercial ammo is available, why bother?
Like its name says, it is a garden flintlock, designed for shooting marauding mice or perhaps cantankerous cottontails. Your energy calculations are not applicable to shotshells, where you generally measure the energy of each pellet, which in your case would be about 1/4 as much as the same pellet in a 12 ga or 20 ga since the velocity is about 1/2 (just that you have a lot fewer pellets).