Thanks for all the good answers. It seems like the consensus is that I don't have to worry about rupturing the tube because the charges are not likely to go off at the same time. The other issue is wear and tear on the internals from all the hot gas and soot from the charges. Since the rocket should need the full 7 sec. delay, I'm thinking of going with two D12-7s and two D12-0s. I'm also thinking of putting a disk of thin G10 on top of the D12-0 motors in the mount to prevent any material from burning forward (as happens in staging) and to provide a little extra seal for the pressurization. (This is what I meant by plugging the motor, not actually tampering with it.)
you'll either blow the discs out or kick the D12-0 casings. You really don't have to worry about all the ejection charges. In fact your fear may end up causing the crash your attempting to avoid. What happens if one or more of you D12-7's fails to eject?
I used to fly a lot of 3, 4 and 5 motor D12 clusters. Clustering D12's is still my second favorite type of flying
D12-0's have a good amount of burn thur gas "Kick" but it's really no were near the volumn of the ejection charge unless you try to cap or contain it. Then it's enough to straighten out standard engine hooks or pop out those G10 caps. C6-0's are the same.
My suggestion to help reduce the charring inside the model is to add a BT-80 coupler just above the motor mount. I've even done this as a retrofit on some of my Upscale as they aged. Most of the Upscale I still have in my fleet have more then 25 flight, some many more others like my 13.25Scale Bomarc right at 25. Shes in for a upgrade from 3 D12-3's to 4 D12's
using the same airframe and stuffer tube.
In most of my Upscale i've never really worried about over pressure, I'm always more concerned with under pressure. usually using a BT-55 or BT-60 stuffer tube leaving about 12-14" of BT-80 as the chute compartment.
3-D12 Ramjet, Laser-X, Aerobee-Hi, 4 D12 Optima, Nova Payloader, Orbital Transport, and 5 D12 Crayon, Hobby Goblin. have never had an over pressure problem. but I did suffer an underpressure NO deploy on the OT causing major damage
Hope this helps a little.