That's a new one to me. I've only seen type XHHW conductors in SER cable, but that's irrelevant. I asked if our local inspector would allow this and he said no. His reasoning in my situation was due to the fact that the cable contained paper spacers in it. I didn't put up much of an argument, in fact THWN was a LOT easier to pull anyway.
In your case, I'd think that THWN conductors pretty much sums it up.
