I'm also looking for a verified high concentration recipe for Test U, though I'd be looking for MCT or Mig840. I've been looking at this for a while however, and it seems unlikely to find one. Looking up the formulation for Nebido (pharma Test U 250 in Castor Oil), not only is it so fucking PIPy that there's an FDA advisory about it, but it tends to crash and on rare occasions leads to embolisms. They use 44% BB (iirc) too to keep it in solution. There's a 2006 paper about using soybean oil instead of castor oil (chinese government bans castor oil products in medicine) to achieve similar pharmacokinetics and it uses no BB and so it seemed promising, but again, it was only 250mg/ml. Though maybe adding BB would let you push it further.
Finally, I've spoken to a colleague who did mouse studies on Test U and his protocol was to use DMSO to initially solubilize the Test U before bringing it to 500mg/ml in corn oil and sonicating it O/N at 42deg C. I asked about crashing out of solution, and he said he would centrifuge it before injecting and never saw crashing (by eye). Though obviously he never monitored the mice for PIP or swelling after.
All that to mean that I will be testing this on a micro batch once I get my raws. I'm not optimistic that classic BB/EO/Guaicol solvents will be enough to get as high as we wanna go, but maybe DMSO would be.