I think I talked to BushHK on the phone today so he knows my take on this.
This post then is for the benefit of the others that incorectly answered your question because they did not know all the specifics.

Barrel nuts generally clear 1.000" inside diameter.
Most 5.56 sized muzzle devices are 0.860" in diameter.
Most gas block ID's are 0.750" ID and the corresponding place on the barrel is 0.750" as well.
Generally, no part of a barrel is over 1.000" except the barrel extension flange.
I'm well aware of DPMS having 1.1" oversized barrels, but these are a special case.

So, if you have a barrel with a permanent attached muzzle device, and the barrel also has a front sight base, handguard cap and barrel nut, there's no way to slide the FSB past the permanent muzzle device.
Now, you can take a dremel tool or die grinder and CUT both sides of the FSB and handguard cap and then the old barrel nut will come off the front of the barrel.
Then you can use a YHM flip-up as you already mentioned.
There are other options on YHM clamp-on gas blocks as well to include the specter (a low profile block) or the clamp-on single/double railed units.
The clamp-on blocks solve the problem of taking apart an upper with a permanent muzzle device.
You will have no problem putting on a freefloat system later once you cut your handguard cap off.