Your thinking is probably incorrect. The wzr-300hp, wzr-600dhp, and whr-300hp is their 'new' line of units, all still come pre-flashed with dd-wrt, as far as that goes, all of buffalo's upper tier wireless routers that have Atheros chipset all come with dd-wrt. Its their newer wireless AC unit that doesn't come with dd-wrt pre-flashed, and i don't suspect it is because buffalo chooses not to support them via dd-wrt, no - i infact believe it would have more to do with the newer AC units being broadcom chipset based units, be that as it may, there would be certain licensing issues that would follow. And they would end up having to pay broadcom big royalties for using their chip and driver with an open source styled firmware, which i don't think either party wants. With Atheros, you have open source with no problem, not so much with broadcom...
As i said, i am not sure about buffalo's release schedule on when they decide to publish builds, and really - if you think about it - with something like this it is not ever certain on knowing 'when' is a good time. Community dd-wrt builds however, get released fairly frequently, its mainly what i use.