I would worry about any threads exposed tearing up the plastic every time the handle is used or binding up.

I like Toms idea of drilling and tapping the OEM to accept a bolt, only issue i see is that the bolt would have to be very small and could bend or break if it spans any joints. Plus the threads in the plastic.

What i would do is get a steel rod of the same diameter and cut threads into the outside of each end so you can put a nut on as a stopper. That way there are no threads under the plastic and you have the maximum diameter possible.

You could probably go even lower tech and just drill a hole at each end and put a cotter pin through to hold it in place..... Or score a grove with a pipe cutter and use a snap ring. There shouldn't be much if any lateral force on the pin, it will all be shear.