The mantra of not selecting flap in a turn most likely stemmed from WWII experience. Aircraft returning with damage sustained in battle did not need to pull any more surprises on their overwrought crews with asymmetric flap while turning.
However, almost every transport aircraft certified in the last 50 years has has asymmetry protection designed into the flap system. Light piston aircraft are not required to have such protection, though several light turboprops have it.
I have yet to read in official manufacturer documentation any prohibition on selecting flap in a turn on any civilian aircraft, be it Cessna or Boeing.
So, it seems this one needs to go to the OWT basket.