[sammyg2]

In order to get a larger diameter drill or tool or whatever in there you will have to drill the holes in the heads larger. That simply will not do.
Those holes have shoulders around them that the head barrel nut washers seat against. If you remove material from those shoulders by drilling the holes in the head larger you will probably not have enough material left to hold the torque on the barrel nuts. I'd venture to guess that the OD of the inserts are at least as large as the OD of the barrel nuts so if you go that big the chances of the washers getting distorted goes way up. The more shoulder the better, the less shoulder the worse. By the time you get a drill and tap and installation tool in there to install the inserts you woulds have to have a pretty big hole. The washers are fairly strong but with constant compression and countless heat cycles I'd be really surprised if they would stay flat.
I don't have a head or threaded insert in front of me right now so I can't measure the diameter of the holes or the insert, maybe someone else can offer up some numbers?

If you remove the heads you "might" be able to get away with drilling the holes through the cylinder fins large enough but if you have the heads off, why not go all the way?
If you have a team of Nasa schientists working on this project with a million dollar budget you could come up with a way to do it, but otherwise color me doubtful.