Pressure loss through holes can calculated using standard theory and loss coefficients. What is important is the length of the slots (or to put it another way) pipes through which the gas travels, i.e. can it be considered a flat plate across which the gas travels in a transverse direction. You also need to consider the angle of respose of the slots, relative to the discharge flow direction, and the compressibility of the gas in question.
It would be useful if you defined what the end purpose is, is this just a problem for college coursework or is it a real world problem where you need to provide a fluid at a defined pressure or flow at the discharge of the plenum.
if you assume the fan to be stonewalled / dead headed then that makes the solution easier as you can then discount the effect of the 'system' on the fan performance curve.
Essentially it could all boil down to Bernoulli and derivatives thereof if you make some assumptions re. ideal gas etc etc.
