Not too sure?
When most people mod their Grado's they remove the buttons from the grills for more air flow. Don't understand why someone would want to restrict the air flow any further on an open back HP? Move the palms of your hands closer to your cups while listening too them, and hear how the sound changes for the worse.
Perhaps there is another way to think about this, is just a thought. I remember the one individual a while ago suggesting a piece of tape across the grill. I tried this with my RS2e and it did seem to give a little more bass. In the case of the RS2e, the tape went across the cup, which sits a bit above the grill. With the 500e, the cup is much shallower, and I would suspect this could cause less than stellar results, but haven't tried it.
I could imagine what the tape was doing is reflecting some of the lower notes back through the driver to the ear. If you take your phones and turn them around, you hear much more bass, as you would with a regular driver in a stereo system. If there is a way to reflect the bass back through without restricting air flow, there could be something there. I can only imagine the math involved to sort this out appropriately though, or a lot of trial and error.
However, one successful example might be the Bushmills. They have the solid back and are vented on the sides. I've never heard these, but if a driver is designed for this, it could work. It sounds like these have a fuller sound, which could make sense if there is anything to this theory (which may be completely ridiculous).
I would compare this to the Qts of a regular driver. Depending on the Qts, a driver may work better open baffle, with out box, or with in a vented box of various configurations, or possibly even a sealed box.
Just throwing this out there as another way to look this. Not intending this as anything more than something to ponder.