I see. So then since you are doing high frequency MOSFET switching, no current driver is needed since fast rise times are not needed since switching is infrequent. You may need a driver, however, if the voltage that switches the MOSFET is not sufficient since a driver lets interface the two. However, in this case (without current requirements) you could use a level converter or whatever else...
Use a 555 timer pulse stretcher circuit (and supply it such that the output voltage is sufficient to drive the MOSFET, but make sure it can also accept the control signal voltages from the fan as well). Have it stretch out pulses it detects, no matter how short, so the period of the output pulse is longer than that of fan's output period. That way as long as their are pulses the output signal will stay high to drive the gate.
There are a lot of unpredictable problems if you try and use a capacitor for this. I'll briefly list some
-delay time turning on
-delay time turning off (like when the fan has stalled)
-frequency dependent (frequency too low, means larger capacitor means increased delay times for the above two points).
-dependent on the average value of the fan's minimum duty cycle
-probably needs a driver of some kind for increased voltage since the fan pulse minimum duty cycle probably has an average voltage far too low to drive the MOSFET.
-because of the point above, analog methods probably aren't going to work, you're better off trying to detect the presence of a pulse and stretching it out at a sufficient drive voltage with a "watchdog timer" of sorts, like the 555 timer method. I can't say what this will do to power consumption.