Standard 12 Volt DC fan for PC: how much AC ripple can support? Does ripple increase noise?
These days, PC fans use 4-wire PWM connection. But the older 2 and 3-wire fans are controlled by PWMing their power supply at a low frequency. This is 100% ripple. Thus, it should work in your application.

I'm not sure about using a single diode though, the frequency maybe low enough for it to vibrate, you'll have to try it I guess...