I don't see them adding this since it would be effectively be an extension of the retainer limit for FC leaders (especially those players with shell FCs for access to workshops or extra housing) even if it was a new type of retainer that could only be used to list items on the MB.

Better just to allow players to continue to deposit gil to the FC chest when they feel like contributing. If an FC is still coming up short needed gil to buy things, it sounds like it needs to get better organized.