I'm pretty sure you can remove the bolt on caps and fit bar end weights (not the stock ones they need the internal fixed threaded bung as stated earlier) or bar end mirrors. May have to lose the throttle boss though. the grip design will minimise vibration anyway and the Rocket is not a big buzzer anyway. I've run aftermarket grips w/o weights on my 1.5" Thunderbike T bars since I have had it without noticing any vibration at all.
Currently have Oxford cruiser heated grips which similarly allow the caps to be removed and bar end weights or mirrors to be fitted. Just picked up some NOS bar end blinkers for AUD$21.60 (US$16) delivered that I will probably never use but you never know.
TIGER EYE Motorrad Handle Bar End Blinkers chrome | eBay