Nick, if you make a hole in the duct so air cold leaks out, you will be wasting energy! Also the cold leaking air will chill the inside of the vapor barrier Because the insulation will not stop the air flow, only act like a filter, which will chill the outside of the vapor barrier, causing it to sweat. You are back to the original problem again. Now the duct is sweating again, ruining the ceiling again. Seems like you are going in circles.
In addition, I can see you have never actually worked on ducts in a tight attic. Insulating with R-6 uses about 1.5 inch wrap. R-19 is about another 4.5 inches. So if the duct is six inch, the outside of the original duct with R-6 is 6" + 1.5" + 1.5" = 9" diameter. Now add R-19 to that which is 10" more. (5" on the left side and 5" on the right side.) Now the duct is 19" in diameter. Hope you have a BIG attic. Especially when the duct gets to the register near the outside wall where the roof slopes down to mneet the wall. I can just visualize you laying in that rock wool insultion in the attic trying to wrap that duct and getting all itchy. Then I visualize you falling through the drywall ceiling. I hope you land on a nice soft couch instead of the floor. If you had ever actually worked in a tight, hot attic, laying in the insulation, You would not propose such a thing.
By the way, you are using duct wrap, are you not? Standard insulation would not meet code when used to wrap a duct. Code also says to follow manufacturer's instructions. If your R-19 bat is not listed for use on ducts, you could get in trouble with the building inspector. The thickest duct wrap I have seen is about 3", or R-8. 2.25" is rated R-6. So R-19 DUCT WRAP would be more like 6.75".
This is rapidly getting out of hand. If you still think it is a good idea, go up in a tight attic tomprrow with some R-6 duct wrap tomorrow and put 3 layers on. Then you will see what hapens when theory meets practice.
Stretch
In short