Here are a couple of ideas. . .The first link is to a pair of socks, which I do understand you don't want to knit. But see the chart for the little chili peppers? Maybe you could knit a guitar strap and then knit that design into it.
Another is that guitarists wipe down their instruments after they play them. Maybe you could knit him some cloths for that, and here's one with a chili pepper design:
The last one is you could knit a wipe-your-guitar-off-cloth with the RHCP logo, the asterisk, in it. You'd need to chart it out on graph paper. What you do it draw it on normal graph paper and then block by block (with each box being wider than it is tall-- they show it the other way), re-draw it on this special graph paper, because knitted sts aren't square the way normal graph paper is.