Chris, my next build is a similar circumstance. Ed Rayl barrel with a patent breech. The trick for locating the vent liner will be finding the antechamber depth and then drilling for the liner.
Ed Rayl told me to not take apart that breech unless it is a necessity, and gave me the afore mentioned instruction.
I agree with you though, the patent breech involves a bit more planning and double checking than a normal breech plug ignition location!
Feel free to reach out if you discover any more info regarding the subject!
Thanks!