The R22 has reached the limit of it growth margins, adding any further weight to the aircraft would required a substantial redesign. Therefore, since virtually all developments will add weight to the aircraft they are ruled out as a matter of policy.
Anyway, adding an overspeed protection device is a small priority compared to the other things that big-R would change if it was worth doing....but of course it is not if you have a complete monopoly on the market (91%) last time I checked. The only customer migration big R has to worry about is customers moving from R22 to R44, hardly a big problem is it, having to take twice as much money off them...
I shouldn't expect too much more to happen to the R22 until R's had is forced...and even then it many not be worth his while, R44 accounts for over 80% of aircraft sales revenue and is in slightly less risky market [i.e. not a primary trainer].
Hope this helps
CRAN