One cannot be a little bit pregnant. But a DB can be only a little bit in the RAM, and specifically in the page cache. SQLite can act exactly like that, and it's damn fast as long as it does not need to durably write a transaction. Polling once a millisecond could spend a few microseconds.

I wonder if using a tiny Redis instance, or even something like LevelDB would be even more efficient.

Thing of the battery!

(read that in the way of "think of the children!")