Well the "corsair freighter" model is actually the mule. A quick check in shiparch.ini reveals the follwing little oddities:

First, the Bloodhound (pi_fighter):

nickname = pi_fighter
LODranges = 0, 80, 120, 1000
msg_id_prefix = gcs_refer_shiparch_pirlf
mission_property = can_use_berths
type = FIGHTER
DA_archetype = Ships\corsair\co_fighter\co_fighter.cmp

And next, the Wolfhound (pi_elite):

nickname = pi_elite
LODranges = 0, 60, 120, 1000
msg_id_prefix = gcs_refer_shiparch_pirhf
mission_property = can_use_berths
type = FIGHTER
DA_archetype = Ships\corsair\co_elite\co_elite.cmp

And now, the Mule (pi_freighter):

nickname = pi_freighter
LODranges = 0, 60, 120, 1000
msg_id_prefix = gcs_refer_shiparch_pirf
mission_property = can_use_berths
type = FREIGHTER
DA_archetype = Ships\corsair\co_freighter\co_freighter.cmp

And, just for good measure, here's the Centurion (co_elite):

nickname = co_elite
LODranges = 0, 75, 200, 1000
msg_id_prefix = gcs_refer_shiparch_hfighter
mission_property = can_use_berths
type = FIGHTER
DA_archetype = Ships\pirate\pi_elite\pi_elite.cmp

So, yes there actually is a "Corsair" Freighter, but it's allready in the game and it's the ugly freighter used by the Liberty Rogues and the Mollys. In fact, all of the models in the corsair folders are actually the pirate ships and all of the models in the pirate folder are actually the corsair ships. Ever wonder why there is no very heavy fighter model in the corsair folder when they have the Titan?