Had a thought about this...have you considered using the wireless driver from your 26b firmware in your next beta firmware for rt-n66u users; i.e. if the firmware is installing on an rt-n66u then use wireless driver "26.b", otherwise use wireless driver "27"? You'll get more testing of new features and fixes that way. I've been using 26b for quite some time now after the one in 27 didn't work well for me.
Or just move the wireless driver back to 26b for everyone, that would be easier, but I don't know if either of these choices is worthwhile for you in moving forward.
Just a thought, wondering if it might help. I'd be happy to test new beta firmware that uses the 26b wireless driver.
Already tried, and after spending two evenings on this I gave up. It's not just the wireless driver that was updated by Asus, but the whole Broadcom SDK, which went from 5.100 to 5.110. That means kernel, linked libraries, newer CTF version with PPPoE support, parts of the build environment, etc... I never succeeded in getting the newer FW code to work with the older SDK components.