Warren, you are correct, plugging outlets to quick couplers will cause no problems. The 584 has an "open center" hydraulic system which means that the hydraulic oil is circulating in a loop and only gets diverted to a cylinder when a valve is opened.
Therefore it is VERY important to have that "L" shaped return line connected to complete the loop.
If the loop is not complete, the only place for the oil to go is through the relief valve which is NOT good. This will happen if you pull the remote hydraulic lever, you will hear the relief valve buzz and the engine come under load.