I have to mention that there were a lot more nonshooting replicas made of blunderbusses than actual proofed shooting versions. Dixie sold a kit way back when, and CVA sold a kit, and a finished gun, but other than those I don't remember any. If this is a wallhanger replica, the barrel may be made of pure junk, which would make it a pipe bomb.
If it is an antique barrel, there is a good chance the brass, or bronze, has degraded over time. So be very thorough in your inspection.
If it is a shooting replica barrel, I would plug the hole, and line the barrel. That seems like the safest solution to me.
I did plug a touchole that was drilled too far back some years ago. I used high heat jewelers hard silver solder, and it worked just fine.
Hungry Horse