Bob, That's some fantastic thinking! How many people said you "can't do that"? Great work!
Investing in European stile mortiser and using floating tenon made of baltic birch plywood complitely takes care of any mortise-tenon application in woodworking.
Plus it is the strongest one, according "Fine
I understand that the baltic ply loose tenon trick works well. I just can't believe that it's stronger that an actual tenon that's part of the rail. Will it support a 10' tall x 5' wide x 2 1/4" thick door? Let's say @ the top of the 3' tall window setup on the 10'' tall door? Are you able to do a strong tenon that'll hold up to daily use? Everthing relies on that stile & the rails meeting it. Can you actually allow for equal movement with plywood? I don't think it wise to use a loose tenon made of plywood for large exterior doors. Most definitley for interior or well protected doors @ standard size. If you look at the tenon pic you'll also see how well it works out for assembling the doors & keeping them weather tight. I also use exterior rated adhesives such as Structan, Excell, Titebond TII. Then I peg them as well. And yes, I egg the holes before final assembly. I do like the plywod idea, though I don't own a slot mortiser, just a little HC type that does everything I throw at it.
With dementions of stock you using, Ican put 4 tenons in the same joint, you using one, it takes so little time with "slot" mortiser. And give incredible strong joint, when it combine with cope-n-stick. Make this tenons a hair thicker, then press them in metal vise, put inthe mortises with glue and they will swell providing joint of inredible quality. Respectfully, ZK