I figured some one would have had this E-ATX thing figured out already
It actually looks iffy, and depends on exactly which E-ATX board you choose.
In the gallery on the NZXT site, the one is shown with a Gigabyte, G1 Sniper 3, which specs at 12 X 10.39, while the Asus RIVE specs at 12 X 10.7, just over a quarter inch wider, which from looking at this review video, could be an issue, where the Gigabyte board fits:
Check at 6:28 and notice that the area for the mobo is not flat with the area where the grommets are, it sits somewhat set in from the surface where the grommets are.
If you watch up to 7:30, you can see a good shot of the mobo standoff and where it sits relative to the rubber grommets.
The pics in this review:
show more clearly how the recess for the mobo tray area ramps out at the bottom and forward edge.
It looks like it would be impossible to categorically say that it will or will not accept an E-ATX mobo, since not all E-ATX boards are the full width of the specification.
It obviously accpets the 10.39" wide Sniper 3, but only a trial would tell if the wider 10.7" RIVE would fit for sure.