Major flight schools such as ATP and American Flyers do it all the time. Many pilot training programs teach commercial maneuvers from the right seat so that a student can take a CFI practical right after getting their commercial certificate. No instruments over there...

Unless there's a specific regulation prohibiting it, I would say it's legal, provided that the PIC is appropriately rated to provide instruction in the given aircraft (IE either MEI or if an appropriate aircraft an ATP certificate.)

That said, it might raise some questions as to legitimacy in an interview down the road (if a career in aviation is your ultimate goal, that is... It's kind of odd to see a private pilot with multi-turbine time...)