I had an idea for a digital meter rather than having a standard bar filling up or a needle pointing to a position on a bar.
The following image will not make much sense unless you read along as well. The numbered text corresponds to the bar's position from the top.
1: Let's say you started a song. This bar is what your rock meter probably looks like.
2: Now you hit every note so far, this song is easy for you so far.
3: You miss a couple of notes into the song, and now your meter is down a little bit.
4: A guitar solo comes up and you mess up pretty badly on it, and now there's a "quiet" portion in the song.
5: You know another guitar solo is coming along, so you use star power pre-emptively.
6: You pass the guitar solo in the low green/high yellow.
7: Let's say you're doing a "tug-of-war" mode now between two teams. You just started the song, and this is what the meter might look like.
8: The song is through the fire and flames on expert, and team one complete screws up the intro to the song!
9: Now team one starts catching up quite a bit, and are now managing to get a streak and keep it going.
10: Team one takes the lead, and team two starts losing.
11: Team two holds red instead of green and misses an entire section of strummed green notes.

As this rock meter is digital, there is little to show that the bar is "halfway" between the highest lit LED and the lowest unlit LED. Maybe you could make the highest lit LED fade in or flash into view. Or, what you might also do, is draw the highest lit LED as if it was filling up, and the excess "spills" into the next LED.
I also had another idea about the 2-player co-op mode's rock meter, but I'll either edit this post with it in or make another post with the idea.
Oh and by the way, I'm back into FoF, I got me a bluetooth adapter, so I can play with my Wiitars now. =D