Or can you?
The problem with creating a key that isn't linked to a specific geomorph is that you don't know how many rooms there will be ; it varies from geomorph to geomorph. But each geomorph could be mentally divided into four quarters, so you could create a key in advance and assign entries to any appropriate space in the given quarter. Of course, that still sounds like prepping a specific dungeon, rather than on-the-fly dungeon creation.
So you don't tie the entries to a specific quarter. Create the entries on index cards and pull four cards for each geomorph. To create the cards in advance, roll 2d6 per card and interpret each d6 according to Sham's 1d6 all-in-one-stocking table:
1 - MonsterYou might want to roll an additional d6 of a different color and add a trap to the card on a roll of 6; always roll a trap die for hidden treasure in an otherwise empty room. Thus, each index card describes one or two encounter areas (and possibly a trap) for one quarter of a geomorph. If a card is completely empty, create a trivial discovery for that card: graffiti, corpse of previous adventurer or monster, random common equipment like burlap sacks or iron spikes, mysterious noises. There are a number of tables out there to roll up this sort of thing, the most recent addition being this d100 table on the Beyond the Black Gate blog, although the table entries tend a little more towards the weird than towards the mundane. Another option is Table I from my 20-sided Quickies tables; roll 2d20 for material + object or action + object, using the result either for an item in the room or as a concept of what the room is used for ("iron armor" might indicate some kind of armory.)
2 - Monster & Treasure
3 to 5 - Empty
6 - Empty (4in6 chance of hidden treasure)
If you're willing and able to improvise off the random combinations of geomorphs and stocking cards, there's no reason this technique can't produce interesting dungeons.