Ive got an old 531 touring frame and I'm thinking of getting a frame builder to replace the drop out with some paragon sliders, so i can set up the rohloff without a speed bone and adjust the chain tension without a springy derialleur thing.. (I will need to cold set the OLD to 135 from 126 as well).
I didn't do it, but I investigated the possibility for protos I built a few years ago.
Spreading the rear triangle sides isn't a big deal, though 9mm is probably the limit of what you should try for DIY cold-setting; you can do it yourself if you do it carefully; instructions on Sheldon Brown's site.
Getting slider frame ends brazed or welded in depends on who you know. Getting the brazing and necessary repainting done for a reasonable price depends not only on who you know, but how near you are to them. I found that it was cheaper to buy a new frame or build the oddities that interested me in wood.
If you find someone to supply the slotted ends, they must also supply actual sliders and demonstrate that they bolt in and slide before you pay. There are lots of version around, but the sliders you want conform to Herr Rohloff's original design. Frame ends in stainless steel, or lined on the inside with stainless steel, are highly desirable.