I have used these extra channels for pixelnet in the past, no problem, with an active hub.
The problem, or potential problem ... is that in the future you may want to move/reassign to a difference range and keep the first 512 for DMX ... I also think its cleaner to do it that way (if you can).
The reason I used these channels was that I was using a pixelnet flashed (dmx) dongle which was limited to 4096 channels and I didn't have enough pixelnet without using some of the first 512 also. At this point I'm using etherdongles with 16k channels each, so I'm moving these pixelnet channels out of the first 512 and into a range thats all pixelnet, just to make it cleaner and less confusing going forward.
Scott