I was hoping somebody else would jump in with specific experience on fighting this battle before. I can only offer an opinion.
My basic position is that it's ok as long as it doesn't directly violate NEC if you wouldn't have been using SKM. Meaning, if NEC methods would have determined that the temperature of the conductors was sufficient for the ampacity at 75C, then you're OK. But if NEC methods would have shown the cable needed to be increased, but then you are using SKM to utilize the "engineering supervision" clauses and go beyond what NEC may have allowed, then I think you're into a grey area and somebody has to make the call.
In the real world, if I felt the length of conductor above ground was sufficient to claim below 75C ratings, I would be ok with it. I'd want it to be well under 75C though. Alternatively, and why I have this opinion, you could put in a short 90C rated splice as soon as the conductors left the ductbank, and then you certainly only have a 75C connection from this splice to your original termination, even if it was short. Would a splice and 10 feet of cable really be better than no splice and continuous 50 feet of cable that went from the original 75C termination into the duct bank? I wouldn't think so.
