Jim,
Precision Rifle makes the breech plugs we are speaking of for the Encore, Omega, and the G2 Thompson Center Products. There has been much discussion about it here at GB. You can do a search a get a lot of posts and info on the subject. I have not seen a plug listed for the Black Diamond. This is probably because the Encore, Omega, and G2 have break open actions and the 25ACP case is held in place by the action being closed.
The Black Diamond looks as if the nipple is exposed when the rifle is cocked. The TC 209 primer adapter for the Black Diamond uses a spring type wire to hold the 209 primer in place. I would think something would have to hold the 25 ACP case in place, like a closed action.
You can go to Precision Rifles website
http://www.prbullet.com/ and look around and maybe ask them, but I feel you may be out of luck.
I use a TC "Dog Bone" that is a red plastic priming tool that easily primes 209 primers in my Remington. When I used percussion caps I used the brass priming tool. I would use one of these tools to prime if I could not convert to 25 ACP.
Hope this helps,
Stan