I think I see your problem, getting the panels lined up evenly across the top on the horiz. piece in the "goal post". I assume you're gonna do the goal post first and then the top rail. If you then make you panels w/ long styles you'd be able to disassemble the goal post and pretty accurately mark the position for the tenons and see the required length for the style. Cut the styls to the right length and form the tenons. And cut the mortises as you usually would w/ a stop block on the table so that you can reverse position and get 'em even.
If that's not part of the plan I also thought that you could use floating tenons.
But then again, every time i think I see the problem and address the issue I begin to not see the problem.
My approach is always to let the piece show me what the measurements should be. This usually works very well unless you're making cabinets, of course. Because, as you well know, to paraphraise the old farmer when asked about his sheep, "sheep lie", he said.....so do rulers.
Sure hope you'll get back w/ you ultimate solution.
Joe T