So, at this point, is there a way to flash dd-wrt with v2.29 running? I tried opening that debug page and logging in with the special credentials, but it reads "wrong password". Also, is what's the benefit/risk of flashing the dhpd dd-wrt official firmware?
Ok, so I found a very simple way to flash other firmware on the WZR-1750DHP without having to do anything besides flashing from the web gui. No downgrading or going into a debug mode, etc. I downloaded a DHPD recovery posted by swallick here (and attached below): https://forum.dd-wrt.com/phpBB2/viewtopic.php?t=282664
(It is a version that allows flashing of firmware seemingly without a version check, used in his case to go to non-dd-wrt firmware.)
I flashed this via the normal procedure as it appears to pass whatever check the GUI firmware update function does. Once this finished and it reboots, you will be running the Buffalo's custom dd-wrt firmware. This firmware seemed to run just fine and had most of the functionality I would expect from the open source version. However, it was quite out of date, so I had the choice to update to a much newer buffalo dd-wrt, or open source dd-wrt. So I went ahead and downloaded the very recent version from the dd-wrt ftp. I flashed that, which went fine for the most part, except at after boot up, it would not get a WAN IP. I did a "restore to factory defaults" reset in the gui, but still no WAN IP. I then did a 30-30-30 reset, didnt come back up for whatever reason, unplugged my modem and the buffalo, waited 30s, plugged in my modem and waited for it to become fully ready, then plugged in the buffalo and upon boot up, all was fully functional.
There may have been some unnecessary or out of order steps that would have made it even simpler, but it was very easy. As 2.29 is almost certainly the last firmware Buffalo will ever release for this device, this should be the only procedure necessary for anyone wanting to flash their DHP in the future.
On a side note, I seem to be getting noticeably better performance from the WIFI and uPnP now works properly (use at your own risk)
I am currently running the buffalo dd-wrt 30357. I am using it as a wifi bridge but somethings it gets disconnected and have to reboot the router. So I hope the new build can fix or minimize the problem.