I would assess the options of grading and/or additions or changes to the existing surface tile system before adding any manner of hard surface to the mix. First of all, no permanent structure should be placed above a drainfield, and your local jurisdiction would be the authority in this regard, secondly, I would not want the required soil preparation and compaction to take place above my drainfield, and thirdly, a percentage (which can vary by climate, soil conditions, etc.) of the water displacement from a drainfield, is via evaporation, and it seems to me that I have read as much as 60%, in which case you could be heavily taxing the ability of the soil alone in this area to absorb the water. I do understand the concept that a hard surface may allow surface water to run farther, but adequate slope of a stable soil can do likewise, and with minimal adverse affects. Just my 2 cents worth.