Ah, and now it dawns on me. I didn't understand the functionality of the Focus Target Modifier. You just get your focus target, not your focus target's target. Derp.

Thanks for responding! Yes, I did all of those thing. I've tried it with heals on friendly targets and attacks on enemy NPCs with no luck. I get the cursor color change, the darkening of some abilities on the bar, and then nothing.

Still looking for a solution to this, if anyone can help!

I've tried a dozen different keys, including all the F keys, all the numbers, and many of the letter keys. The checkbox in preferences to enable Focus Target Modifier is ON. When I press and hold the focus modifier, the abilities on my hotbar go dark and the mouse turns purple, so I know it's 'functioning', but when I then press the hotkey for a button on my hotbar nothing happens.