This functionality can now be added by applying the latest fixpack (fix 47).
By setting ex_rank_gunship_next to 0, a player can have the WMD gunship once. From that point forward the code will only assign airstrikes.
After applying the fixpack, please change the following variable description in ranksystem.cfg to reflect the new setting:
// How long till the player is allowed gunship again
// 0 = disable (gunship once, then only airstrikes)
// 30 - 1800 = delay in seconds (default 60)
//set ex_rank_gunship_next "60"
To apply the new "gunship once" functionality, set it to 0:
// How long till the player is allowed gunship again
// 0 = disable (gunship once, then only airstrikes)
// 30 - 1800 = delay in seconds (default 60)
set ex_rank_gunship_next "0"
This only applies to the gunship linked to the ranksystem (ex_gunship "2").