Thanks for the help so far. To achieve my desired result, it appears as though I can create a new player class that extends EntityPlayer, and from there override onLivingUpdate().
The part I'm having trouble with now is how to register my new player class and get it to take the place of the normal player. For blocks, items, and tile entities, it is a simple matter of calling GameRegistry.registerXXX, but I don't see any registry for the player. The closest I have found is EntityRegistry.registerModEntity, but that seems to be for mobs only. As usual, any help is appreciated.
p.s. I appreciate these forums, but the search function is utterly worthless. I tried searching for "Register Player" and other similar phrases, but the results were nonsensical, unrelated, and restricted to only a single page. Are the operators of this site aware of this?