I think one of the core concepts here is getting an event where crafters and gatherers are active participants, whereas Besieged is basically pure combat.
"Besieged" in FFXI isn't limited to the battle (even if it is the main event) : the power of beastmen are determined by some factors influenced by players (beastmen killed, astral mirror destroyed, etc.) and it's easy to imagine what crafters could do in this system (repair buildings, repair weapons, make medicine, etc.)
And it's actually what happens in "Campaign", which is the better system.

Btw, we need a campaign-like system in FFXIV, because it's one of the few concepts that can actually legitimise the "MMO" appellation.