What I was thinking is a 2x2 brick that you put the items on. Since the idea would be that you put them in a room away from the featured map, perhaps when they are planted and get a shapename of "Rarity: 1 (Ubiquitous)" (1 being ubiquitous, 5 being very rare), and you use an admin only randomizer rarity tool (alliteration: Randomizer Rarity Wrench), probably an orange recolored wrench or something to decide its rarity.
Perhaps you just click the brick and it says
"\c6Press a number \c31\c6–\c30 to set the rarity for \c3<ui name>\c6."
Press a number 1–0 to set the rarity for <ui name>.
1 (Ubiquitous)
2 (Common)
3 (Uncommon)
4 (Rare)
5 (Very Rare)
What do you think? Is this a good solution on the user end?