I've been looking at this for awhile and I have an idea. It's basically along the lines of Thias' idea; repair it from the inside. Anyone please let me know if they know for sure that this is infeasible.
It looks like the tubes have moved slightly apart where they're cracked. It occurs to me that maybe you could position the frame in a jig that would hold everything stiff and still allow you to move the tubes back to exactly where they should be by using some kind of fine-adjustment cranking device (without causing further cracking, of course).
Then try to peer thru the top bottle mount hole to see if the elevated stay tube can be accessed, and if not, I'd bet you could get a long skinny drill-bit that would allow you to drill into it (so that air, water, etc could pass from the stay-tube to the seat-tube). If there's already a hole there, ream it out so it's bigger and THEN mix up a bit of carbon fibers with some epoxy and apply it via the top of the seat-tube into the area of the crack. If it was a goopy enough mix, you could simply pour it down the seat tube. If the mix doesn't flow, you could use a wooden dowel (a broom handle?) to tamp it down in there from the top of the seat-tube like you're loading a squirrel musket. Perhaps there's a way you could restrict the goop from moving more than 5 inches past (underneath) the stay-tube/seat-tube junction by using a temporary (removable) plug, maybe by using a balloon or even something you could dissolve later in water, like papier mache'.
The epoxy curing process would no doubt create some heat, so you don't want to do a whole lot at any one time, otherwise you might affect the paint. After the first application has cured (or even while it's curing), mix a little more and tamp it down in there, and keep repeating the process. Little by little you could effectively fill about 5 inches past the T-junction where the 2 tubes meet so that eventually you have a solid T-shaped piece of carbon fiber "plug" that completely fills the space inside the tubes about 5 inches into the stay-tube, and about 5 inches below and 5 inches above the seat tube crack. I can't tell from the photos, but perhaps there's a vent hole somewhere in the stay-tube that would allow you to inject an aqueous mix of epoxy so that it would flow to the seat-tube junction? You'd have to tilt the frame and allow gravity to direct the flow, of course. My thoughts are that using this method, you could layer the epoxy into the tubes, starting from the bottom (about 5 inches below the cracked area), and work your way up to about 5 inches above the cracked area, and you'd still have plenty of space for a seatpost when you're done.
After you've created the carbon fiber & epoxy plug, you could use a teeny bit of glazing putty to fill any noticeable parts of the crack that still exist on the outside of the frame, and then touch up the paint so that it looked as good as new.
I realize that sounds like a lot of work, but it might be worth it for such a rare frame. It might not ever be strong enough to thrash (I suspect it never was), but I'll bet you could save it and make it a ride-able frame. That said, however, I have no experience with carbon fiber, so it's possible I'm completely wrong (and I encourage anyone with experience to comment either positively or negatively), but it occurs to me that it might be a viable fix without replacing tubes, re-welding, adding gussets, etc., so it would appear original on the outside.
So that's my vote. Try to save it and ride it (gently), and the wall-art option will still be there for you if you ever change your mind. Best of luck!
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