http://www.woodshopphotos.com/albums...m/mullion3.jpg
How about this?

Make C and D thick enough to fit the groove plowed in the front mullion
pieces (A, B, E, F) plus the thickness of the glass and the stop which
isn't shown.

Glue C to A and B and all the other As and Bs. Glue E and F in place
then glue D in place.

Pieces A, B, C & D can be grooved in long strips. Then the miters cut.
As you say, this would be trivial.

Hope this works for you.

Dave