The Shadow of Yesterday and Keys

Motivating character behavior (and Arcs) with Keys.
I like.
Something that encourages roleplaying with a nice carrot, but that does not straightjacket the player.

Clinton R. Nixon's The Shadow of Yesterday introduces the notion of keys as a way to motivate in-character behavior:

Keys are the primary method of increasing a character's abilities in The Shadow of Yesterday. These are goals, emotional ties, or vows a character has. By bringing these into the story, the player gains experience points (XP) he can use to advance the character, increasing pools and abilities, or learning new Secrets and Keys.

Keys also afford dramatic character arcs:

Whenever a player has a character perform the action shown in one of the Buyoffs, the player can (this is not mandatory) erase the Key and gain 10 XP.

I can see easily importing this mechanic into any number of games. Has anyone given it a go yet?

I may try this in future games I run.