The reason I'm considering something like the QUBE is that it's UL rated for grounding, and looks like it grounds stuff. I can easily hang a green wire out of a plug I bought at Lowes, but there's going to be some AHJ that questions how you wired it. In this case it should be obvious that the ONLY thing it can connect to is the EGC contact in the NEMA receptacle. Note that one of these QUBE's will feed "grounds" to three different circuits.
Answering another post reminded me I wanted to comment on this. The QUBE (while apparently a very useful device) does not "ground" anything. It provides a way to "bond" amplifiers/gear/whatever to the building grounding system.
On the surface a pendantic observation-but the difference between grounding and bonding is key to understanding what needs to happen for safety sake. Perhaps a topic for Code Study, if and when we get that off the ground? (Sorry-that was intentional
-caught it after I typed it but gonna leave it!)