TWA operated two Jetstars for advanced pilot training back in the 60's. The panels were modified to replicate the B707 and B727. I think they we found to be cost prohibitive rather quickly.
Kelly Johnson of U2/SR71 fame had one of the first two JetStars assigned to him for a few years in the 60's. The first two JetStar prototypes were twin engine as to opposed to the final 4 engine configuration that evolved.