FFXI will always change!

[Hyaki]

You think your idea can work is just because you really don't understand how the porter system works, nor try to learn.
If you ever tried to design a data structure, you know what we are saying: One porter slip can hold data in a number of hundreds... maybe thousands times of one keyitem can hold, that means SE need to design a same amount of keyitems (as the amount of equipment types accaptable by the porter system) in order to implement the storage system without using current storage slip.
That's also the reason when the armor storage system first implemented it can only accept a full set in one transaction.
The system can work that way because it doesn't need to change any existing architecture.

In one line: Turning the slips into keyitems is TECHNICALLY IMPOSSIBLE without entirely redesign the keyitem architecture, unless you want a keyitem for each single one piece of your equipments.
The idea is good for a player, but it might never be implemented due to tech limit.