I agree with Carl, all sideloader plugs in Rifle (cylindrical-bodied) shells were lead.
I also agree that pitch/asphalt is the most common case-shot matrix.
You asked "Did they use a brass [side]plug any?" Yes, some Confederate Bormann brass underplugs were salvaged and used as sideloader plugs. Those are slightly less rare than the copper sideplug.
About 3 years ago or so, Selma Hunter came up with a half-dozen 12-Pounder sideloaders from the river at Selma, which had a screwdriver-slotted brass sideplug, if I recall correctly. But I may be remembering wrong about the metal. If it was copper, I hope he'll speak up to correct me. I think Emike knows for certain.
Regards,
Pete