I decided to make an AutoHotKey script to be able to play FFRock with a guitar. I've even set it up to require the player to strum (which just had to be much more difficult to implement). Granted, I'm not very familiar with AutoHotKey scripting, but my knowledge of programming and some tinkering got me through. I doubt this is the most efficient or *best* method, but it does work.

NOTICE:
Put this script in a new AutoHotKey Script, and save. If you aren't familiar with AHK, see this thread - http://www.flashflashrevolution.com/vbz/showthread.php?t=41727.
YOUR JOY/KEY SETUP WILL PROBABLY BE DIFFERENT. Anytime you see #Joy# (where # are numbers), change these to appropriate values. The first # is the joystick number. The second is the number representing the button to be pressed (Y represents the Y-axis, the strum bar, fyi. Don't change it). To get these values, you'll need the JoystickTest Script, which can be found through a link in AHK's help file, as well as how to use it.
So here's my script:

Loop
{
GetKeyState, joyy, 2JoyY
if (joyy = 100 or joyy = 0) ;If strum axis is up or down
{
if(Strum = 0) {
Strum = 1 ;Only one set of 'Send's per strum
if(GetKeyState("2Joy6")) ;Green Fret
Send {a}
if(GetKeyState("2Joy2")) ;Red
Send {s}
if(GetKeyState("2Joy1")) ;Yellow
Send {d}
if(GetKeyState("2Joy3")) ;Blue
Send {f}
if(GetKeyState("2Joy4")) ;Orange
Send {g}
}
}
else
{
Strum = 0 ;Reset Strum variable
}
}

1::Pause ;Like Suspend - stops sending keys

(I may improve this someday, to auto-detect controllers or something. This was tested with a PS2 controller, so I don't know if 360 etc. controllers will work or not, but it shouldn't be a problem switching over.)