This does not work if you have flashed to 2.27
I can confirm this worked for me on the EU/UK, RUS and US routers in my possession when moving from 2.27
Please don't post things as fact when you may be the exception.
As it happens i am looking into the possibility that some US routers do not update using the Factory-DDwrt Firmware. This issue has also been reported by Leonard.
At present, i've worked out that people who have new routers with 2.24 or older, that update to 2.27 then use the factory-ddwrt release (like myself) will have success.
However, those who start with a US router 2.25 or later then update to 2.27 before trying factory-ddwrt releases will not have success.
This indicates that there has been a minor hardware revision OR something was added in the 2.25/2.26 release that acted as a precursor to the 2.27 restrictions. The fact that it is also limited solely to US routers at the moment (until someone reports otherwise) also would signal this hypothesis as correct. I've also had reports of Grey Market Import models (Which inherently are also US internally) have also had some problems.
I'm trying to ascertain what is going on.
But beyond yourself and Leonard, i am not aware of any more than a handful of people who can't update using the decoded firmware or by using a hex editor to modify the webflash releases (which makes them effectively the same as the factory-ddwrt release anyway).
In the meantime, i really recommend bothering tech support in your region; As i have said elsewhere, the more people who bother tech support, hopefully the sooner Buffalo will look to resolving issues with their licenses. Plus, they've already set a precedent by helping users out in reverting/changing from 2.27, so it is possible by legitimate means.
After-all, for a router that costs £120/$190 you would hope that buffalo would have enough cash to rectify whatever problem there is. If not, they should reduce the cost of the router IMHO as their marketing is pushing a product that since the 2.27 update is not as flexible as they suggest. It's also interesting to see that the WZR-1750DHPD, that includes the DD-WRT firmware by default has all but disappeared from sale in the UK and is increasingly rare in other countries so i imagine that we are about to see a whole host of Buffalo-Firmware only restrictions as new routers appear.
Not a situation i think any of us want to see, but it's increasingly looking like its going to be the case until people start bothering the manufacturers en-mass to sort out flexible licensing for their firmwares.