The only blue restrictions I can find on "one action per keystroke" are:
1) No automated gameplay (loosely defined on purpose)
2) No doing multiple things in an external macro (e.g., G15) that you can't do in an in-game macro (example: cast fireball, wait 1.5 sec, cast fireblast)

They have definitely ruled that multiple keystrokes are ok if you are sending a string like "/follow" from a G15 key. AFAIK they have never ruled on whether multiple actions that don't circumvent the GCD are ok. For example, I have the following bindings on my n52 d-pad:

D-Pad N = w
D-Pad W = q
D-Pad NW = w + q

Is that a violation of the TOS? I'm pretty sure that they would say not -- all I'm doing is implementing a natural scheme for moving around with the d-pad. I'm not gaining any unintended advantage over other players or circumventing the restriction on multiple actions that trigger the GCD. I just get to use the d-pad for movement instead of WASD(QW). Likewise, I think it's ok for me to trigger "q" + "/" off a single press of a mouse button because all I'm doing is combining 2 non-GCD in-game commands to get an effect that is trivial for a single-boxer to achieve with a keyboard and mouse.

That's my take anyway. I doubt that we will get a definitive ruling from a GM unless multiboxing becomes a lot more common. Right now there aren't enough of us for it to be worth their time to create authoritative rules.

If you do find a blue post that's more definitive than what's above, *please* linky back here I think everyone on this board wants to stay within the rules. We just wish we knew what they are! :P