the porting is done when the barrels are made. The barrel manufacturer simply over size that 4" when doing the initial bore drilling (before adding rifling), that process adds one extra drilling step that is all. side holes can be drilled in a helix pattent so there is no index requirements, 3 rolls, 6 holes per roll is plenty. again done by the barrel manufacturer.
Now a threaded muzzle break with precision machining retails for about $60, the cost to make it is probably only $30 (with all that precision threading and such). The proposed HP porting is done directly on a barrel and the only real possible trouble is drilling that 18 holes, but that should only be a 5 minutes operation on a vertical mill, just clamp the barrel into a index head.
Plus, I suspect the HP barrel itself is a <$30 part, adding $20 extra should get you a lot of machining.
All the real "mod work" will be done by the barrel plant, HP just pick up the barrel and assemble it as usual. No process alteration for HP.