Bunker buster behavior is not hardcoded, although it is not very flexible as well, but what you seek is possible.
I wonder if you've seen this:
Code:
Behavior = BunkerBusterBehavior ModuleTag_04
UpgradeRequired = Upgrade_AmericaBunkerBusters
DetonationFX = FX_BunkerBusterExplosion
CrashThroughBunkerFX = WeaponFX_BunkerBusterIntialImpact
CrashThroughBunkerFXFrequency = 571
SeismicEffectRadius = 200
SeismicEffectMagnitude = 5
ShockwaveWeaponTemplate = BunkerBusterShockwaveWeaponSmall
OccupantDamageWeaponTemplate = BunkerBusterAntiTunnelGarrisonWeaponWithABigName
The last line of this indicates a weapon that applies damage to units which come out of the tunnel. Change the value of this parameter with "RangerFlashbangGrenadeWeapon" and you'll have all vehicles which come out of the tunnel not even scratched. On the other hand, change it with a name of a weapon with "ARMOR_PIERCING" damage type, and you can have vehicles dead and infantries out.