I don't believe you've answered Able's question on the hinge design, nor mine on where the out of plumb side is relative to the doors, Bob...

For example, if the doors are anchored on the left edge, hinged in the center, and the right jamb is the one out of plumb, then you could just trim the right edge of the right panel to match the jamb angle. (Or reverse this if applicable.)

Edited add - Bob, I'm curious as to why you can't remove the casing and shim the jamb out to plumb?

The 24-1/2 is too large for that door (which you already mentioned) adding 3/4 to each side will reduce the opening to 23 inches. Too small for the door. I'd be inclined to use a 1X2 on each side that will be installed flush with the front of the door face then cut the door to fit with the gap 1/8 out of plumb from top to bottom.

[edit] or shim the 1X2, then add 1/4 round to cover the gap and also to the other side. That to me would look much better than doubling the jamb.

So you'd fasten your 1x2 to the door panel itself? Then trim it to run parallel to the out of plumb jamb? Then refinish the panel? (Assumes of course that the out of plumb jamb is the one opposite of the bi-fold anchor hinge side...)

No, build the existing jambs out with the 1X2 at the point where it's flush with the door panel. If you shim the 1X2 on the out of plumb side use qtr round on both sides. If you don't shim it plumb, cut the door (you have to cut the door in either case) to divide the out of plumb 1/4 inch .

I believe I'd shim the 1X2 and use qtr round. That would look a whole lot better (IMO) than doubling the whole 4-1/2 in jamb.

The hinge side is plumb, the latch side is out the quarter inch.

The doors are hinged together with non-mortise hinges and the same style hinges will be used to connect the panels to the jamb.

The doors, existing jambs and casings are all custom stained by the customer who doesn't wish to disturb them. Yes, she'll have to stain the new jamb pieces.

With a thickness planer, I can thin down some 1x poplar to about ⅜ " on the hinge side to make half of a 'filler'. It's the other side where the filler will need to be tapered to accommodate the out of square issue.

Bob.

potential heresy but there should be enough adjustment available in those doors to make up for 1/4 inch over that run. ront

Instead of trying to taper 3/4" down to 1/2", plane the whole piece down to 1/2" and shim to make up for the 1/4" where needed. Then trim with molding and everything will be square.

Sure would be nice to see photos Bob...

Update-
Everything came together today to work on this project.
I talked to the customer a few weeks ago and convinced her that padding out the width of the jamb was a bad idea. So, we decided to shim only behind the edge of the bi-fold door panel.

When I got my framing square on the door, I found that the top jamb was not level and the hinge side was not plumb (contrary to my earlier statement). The latch side was plumb.

So, I fitted a ¾" x1 ⅜" spacer on the hinge side jamb, just outside of the old built in 'stop'. Then, holding the door panel assembly against the latch side, I scribed the top and hinge sides, took the assembly down and used my power plane to shave down the panels to the scribed line. Mounted the hinges to the panel and hung the assembly in the opening. It looks fine as the 'adjustment' on the panels is hard to detect.

Need to tweak the stops a bit and mount the latch, but, the hard work is done.

Thanks for all the great ideas, Bob.