They are starting to do some long range within the NMLRA. You might snoop around there or go to their facebook page. It is open to all.
I do know a lot of the guys shooting bullet guns use Platinum lined nipples. It helps to control erosion of the flash hole.
Slugging the bore might be a challenge. Easy to get the bullet in, getting it out with damage is another story. I would start with a soft cast .457 ball and use a CO2 discharger to push it back out.
What twist and what Pedersoli recommend?
Good luck with your project.