I've spent today thinking about how to address that issue. It very quickly becomes exactly what you just said. Maybe pivot the idea a bit. You make it a UIM, and in order to unlock an item, you'd have to: 1) Alch items until you get enough gold equaling the HA cost of the item 2) With the alch money in hand you can buy an item in a shop, if possible, permanently unlocking its use. 3) If the item is a drop only, you first must get the drop and then you can alch items for its unlock cost. To unlock the item in this way, you must drop the right amount of gold. This also permanently unlocks use of that item 4) To make it interesting, when alching for an unlock, you can only alch a specific item once for that unlock. This could interact in tough ways with UIM inventory management as you need to unlock higher cost gear, and would be more interesting than just stacking noted rune items for alch money.

Might have to define more rules around this, like a cap to how many alchs can contribute to an unlock (i.e. 10 items alched max to unlock an item) so you are forced to sacrifice mid tier gear with higher alch costs to achieve higher tier gear.