I am a fan of well executed mid-century modern. IMO, I would not add anything on top or around the miter to "protect it". The MCM asthetic is punctuated by it's simplicity (sometime deceptively as you are finding out with such a long miter joint). If you are concerned about durability, you can (and probably should) chamfer all the corners slightly. I would not be too concerned about the wear on the edges as I have seen a lot of modern designs that have held up well. I hope you keep us posted on the build, I'd like to see it coming along.
You should ignore my textile suggestion. I didn't know you were doing "mid-century modern" stuff. It doesn't suit my tastes, so I don't know anything about that style. I assume glue blocks weren't used in the factories that made the original. Let us know what you figure out!
Your endgrain is like your bellybutton. Yes, I know you have it. No, I don't want to see it.
Ask me why I use hand tools, and I'll tell you
The catch is that in order for this to work, you have to veneer the case and use a stable substrate. In my case, I use baltic birch plywood. Others have used MDF, but while MDF will be OK on a small box, it's debateable in a full-size case.
Here's an example from the FWW site (the article actually mentions the that the purpose of the edge-banding is to resist wear, in addition to aesthetics) :
That is a very cool design and now what you are asking about makes complete sense. Certainly the joints will need strengthening. Could a mitered spline joint work here? Of course it wouldn't be all the way thru the joint, end to end since then the spline would be visible. I'd certainly want to add the blocking inside anyway, but that would be cross-grain, wouldn't it, I mean, continuous blocking. Would the blocking be screwed inside and then glued to each other?
As has been mentioned above, you'll need to somehow reinforce the miter of this length.
It can be cut by hand, if you can crosscut a square this long (which is ambitious) and
layout your 45 degree line to be cut with a plane. Any longer handplane with a blade wider than the
exposed section of the miter will do - so long as there's minimal camber.
I think you could do it with a well tuned jack plane.
You would need to back up the exit of what amounts to a long chamfer to prevent blow out.
Mount a sacrificial upper and lower rail of wood that's harder than your stock to give the plane sole something to ride on, as a depth stop.
If the corner of your rails is not cut by the blade it would guide the plane sole along the length of your workpiece.
(I envision a lower rail tacked to your bench, the carcasse side above that with a spacer as thick as the lower rail, the upper rail applied with double sided tape and a third rail to act as a limit atop that.)
If you've ever seen a bowling alley with gutter guards, that's the idea I have for your problem.
Presuming you're right handed, the right hand side of your plane rests against your bench.
The middle of your plane is suspended over the work piece to be cut.
The left hand side of your plane would be up against the upper rail "bracket".
If that works (and I can't say I've tried it) you would still need some kind of cross grain spline for strength,
as in a picture frame.
The photo shows the rough idea, but the workpiece needs to be raised off the bench to reach the "lowest" corner.
I just watched Tommy Mac do that on his show over the weekend with some thick ambrosia maple. You might look for that episode.
An angled shooting board will be the best way to trim the miter. You will definitely need some sort of spline or other reinforcement in the joint. You don't really need to worry about damage to the corner. It may pick up some minor dings, but it won't show end grain. The bigger issue is that it is a pretty sharp corner. You probably want to blunt or round that corner slightly.
A lot of good options have been suggested, but since you find the idea of shooting a long 45 mitre scary, it seems that elegant joints like blind mitre dovetails are not realistic now.
First off, it isn't that hard to cut a long 45 mitre. The boards must be very flat. Layout very carefully. Cut back from the line a few 1/16's, then shave to the line with a very sharp plane, a block or other low angle plane will work. An accurate jig will help a lot. Pay close attention, take very fine shavings as you approach the lines, and make certain you don't cut past the lines.
Will She Who Must Be Obeyed permit splines? If so, they are easy and foolproof. Just glue up the corners (let the glue soak into the endgrain without drying, then apply some more just before closing the joint) and hold the carcass together with packing tape on the mitres. Practice this with dry joints first. The tape will suck the joints tightly together. Makes sure the carcass is square and the joints are tight. Then use some more tape on the diagonals (X's) at front and back openings to maintain a rigid frame temporarily. When the glue has reached strength (min 24 hrs), layout the spline kerfs and cut them by hand. Glue in the splines. When they are dry, trim and shave as necessary.
If splines are too much, and the design permits, full-width glue blocks with screws will work too. Not elegant, and you might need to design the doors and drawers to accommodate them, but they will provide good strength.
Send me your email address and I'll send you a PDF of Rolf Shutze's "Making Modern Danish Furniture." You also might consider buying Charles Hayward's "Modern Furniture Construction."
It's a great style and great fun and very popular again! Go ahead and burn some electrons. If you can pull off THE definition of brutal woodworking: 23" times four sides in secret mitered dovetailing then feel free to put me on ignore.