Email the MLAGB. They have cannon shoots and may be able to tell you if the ball is patched. They may also be able to give you some advice about where to use it as well.

As far as I know you don't use a patch on a cannon ball I think you use wadding between the powder and ball. I could be wrong though.

I like the sound of an 8 bore brass cannon. I wish they had not been mentioned I might be tempted to get one.