Maybe something like the following could work?
PHP Code:
// SDKHooks required here
public Action:OnTakeDamage( victim, &attacker, &inflictor, &Float:damage, &damagetype, &weapon,
Float:damageForce[3], Float:damagePosition[3], damagecustom )
{
// TF_CUSTOM_ROCKET_DIRECTHIT found in tf2_stocks include file.
if(damagecustom == TF_CUSTOM_ROCKET_DIRECTHIT)
{
// Someone got airshot! (I think... Haven't tested this...)
damage = 9001.0;
return Plugin_Changed;
}
return Plugin_Continue;
}