Some bad news. Initial observations from last night I streamed a movie through plex at 1080p, and the slimport with attached OEM charger and cable I was very slowly losing battery. I want to do more testing, but it seemed like the slimport either took some of the current for itself or the Nexus7 was using a tremendous amount of it. I will do a test later tonight that can measure the current throughput to ensure that it's charging at full current.
I did find, however, that with the slimport attached (without microusb charger plugged into it), you CAN wirelessly charge, but I found it to not keep up with the demands of plex and the attached slimport, just like the standard charger. Note when I was doing this test I only had one charging method active at a time, either wireless or through the slimport charging port.
I will do more tests, but it looks like in general there might not be a way to perpetually output to hdmi through slimport, as I'm not finding a charging method that can keep up with all scenarios, mine using plex as an example streaming 1080p