Plenum Return Systems

Hi All, Compliance with this credit in fully ducted return systems has been straight forward, however, the LEED Reference Guide is not too clear on how exactly open plenum return systems are to be addressed. The Reference Guide states that "If an unducted plenum over the construction zone must be used, the team should isolate it by having all ceiling tiles in place". It is not clear if the MERV 8 filtration media should be installed at the return air grilles, or at the return duct mains to the air handling unit. It seems that placing the MERV 8 filters at each RA grille may not be the best solution, as the plenum will still accumulate dust during construction and during occupancy. It seems that the more prudent and cost effective solution would be to place MERV 8 filtration media at the return air mains to each air handling unit. Does anyone have experience with this? Thanks!