Okay, so I pulled out the tape measure so we have actual numbers. The stock barrel is 17.7" long with the portion sticking out of the breech 15 11/16" long. Meaning that 2.0125" is inside the breech. The barrel band starts at 5.75" from the breech and is 1" long, ending at 6.75" past the breech. Meaning that the barrel band starts at 7.7625" from the leading edge of the barrel and ends 1" later. So, on a 28" barrel, there would be 19.2375" sticking out past the close edge of the barrel band. Or 19.7375" if you want to measure from the middle of the barrel band.
The ID of the barrel band is .78" with the OD of the stock shroud .75". The wall thickness at the thinnest part of the barrel band is .121", just under 1/8". I don't know if that leaves enough material to cut a groove into.
The other thing I noticed is that there is ~1/32"-1/16" milled off the bottom of the stock shroud to give it a bit more clearance to the air tube. Rotating the stock shroud shows that there is maybe 1mm between the airtube and a full .75" OD shroud or barrel. I don't know that it's a big deal, but it might be.
So, I don't know if there is enough material to machine an o-ring groove in the barrel band nor do I have the skill/tools to do so. I could find someone to do it at a cost. I don't trust the wall thickness at the thinnest part of the plastic barrel band to hold a grub screw. I could tap one screw on either side of the barrel going up at a 45° angle to push the barrel up against the top of the barrel band I guess.
At this point I'm starting to really wonder what length of barrel past the barrel band you would trust a 9/16" barrel with a .633" OD CF sleeve to stay stiff with the grommet mod that I've done. Using a density of .284 lb/in3 (quick Google search on the density of chromoly steel) a 9/16" barrel that is 10" longer than stock only adds .568 lb to the weight of the barrel out front, when accounting for the .25" bore down the center. I don't think the extra weight of the barrel is the problem, so it must be the axial stiffness of the steel at that length. Am I on the right track?
Here is a picture of the grommet mod. Basically I have a full 1" of rubber sandwiching the barrel in the barrel band quite snugly.
