More useless rambling on my part:
Hmmm. Some JBL 15s, huh? Personally, I would not find anything concerning in mounting one of those fifteens using the twelve-inch baffle board, despite blocking an inch or so of the speaker circumference. What you would be losing is basically the surround area of the woofer. (And you will need to check to make sure that the surrounds do not buzz against the baffle. Easily solved with an extra gasket on top of the factory job. That will move the woofer further from the baffle.)
You could give this a try very easily and inexpensively by simply installing four (or eight) brass threaded inserts per cab to hang the fifteens on, and you wouldn't have to modify the baffle at all. (And, the pilot hole for these doesn't even need to protrude through the front of the baffle board, so no outward appearance alterations and no holes through the baffle. Neat.)
Ok. So if you do not have any components on hand to use for the HF section, what did you have planned for that? (Of course, going with a coaxial speaker would take care of the woofer and tweeter issues in one unit.)
But, I'd try the 15" JBL woofers you already have in those cabinets before going any further. If for nothing else, just the grins involved. You may be surprised. One way or the other!
GeeDeeEmm