There's no way I would personally attempt it, because I just get to scared about stuff like that.
But if I was gonna do it, here's what I'd do..
1.Stuff the rag in there like you said.
2. Make sure the valve for the port you are working on is closed, so just incase any shavings get past the rag.
3. When your done with each port, go in there with a strong telescoping magnet and get out all the little stuff that the shop-vac didn't get.
You will probably have a hard time getting a good angle with a die grinder under the hood. The bumps take a long time to grind off, I know, I was using a very powerful die grinder with a brand new carbide bit with the heads on a bench and it still took a very long time. I can't imaging bending over for that long.
I don't believe the gains you'll see from it will really outweigh the time and pain you put into it, because you aren't doing a full port job.
Good luck with it though, let us know if it's still alive when your done
