There are actually test betas on the community dd-wrt file server (build 22118) for all the newer buffalo AC units (wzr-1800, wzr-1166, and wzr-1750), whether or not they work for the 2 latter units i listed is yet to be seen as no one has reported 'yay or nay' within the forums for those models but for the wzr-1800 it is confirmed to be working, albeit with some caveats - which i suggest you read and research in the dd-wrt forum on.
As far as official buffalo dd-wrt builds for these units is concerned, i highly doubt there will be any due to the licensing issues that would ensue. Broadcom isn't an open source architecture, so i'm sure they would want big monies for something like this, some of the other buffalo units have official buffalo dd-wrt because they are Atheros base units - which is pretty much an open source platform, so no big monies to fork out for its driver development.
And to answer the last bit, the stock firmware on those units ('user friendly firmware') is pretty much like the one you see for the G300NH, with a few new feature additions, but not near the functionality of dd-wrt, as far as that goes - i have yet to see any kind of stock firmware with any feature set comparable to 3rd party firmwares.