Is that even possible? From the looks of it, OP2 considers the GP as a building, and as such, is subject to the damage limit, unless that can be easily changed to something else (ex 0) without the game crashing...
To be honest, I don't think GPs should get a boost. You shouldn't be able to just dig in and become impossible to attack (SupCom would greatly suffer from this if Experimental weapons didn't exist).
Guardposts shouldn't be able to hold their own against an attacking army. They should be used to deter flare/novae sneak attacks, and to supplement a defense, even acting as a deterrent to small raids, and for this, they do the job admirably. I really don't think a change to their functioning is necessary.
Also note, that a properly upgraded GP, with tubes, is about as powerful, if not more so, then a panther. (Heavy armor, +50% damage, +25% damage reduction (Dirt), Increased sight range)
I know the last one doesn't help much with fighting, but it certainly helps provide a second or so more of warning if you happen to spot a night sneak.