Thanks for providing the links to firmware version 6.1.4.3.
To my dismay, I did notice that when I tried to use it with DMT, the checkbox unlock experimental area Panel/Last Tab still remained greyed out.
Accidentally, I found the cause and the solution.
The Cause: I had previously downloaded and used DMT with a later version of the firmware (post 6.1.4.3). Of course, the unlock experimental area Panel/Last Tab was greyed out as Thomson had locked out the feature. But when I downgraded to version 6.1.4.3 the option was still greyed out. It seems that because I had used DMT with a later version of the firmware, then this later version gets remembered somewhere in DMTv7.35.ini - I'm not certain where exactly.
The Solution: I happened to be testing the ST585 on another computer where I had not run DMT before. When I downloaded a fresh copy of DMT and ran it for the first time, it created a new DMTv7.35.ini. Lo and behold, this did now work and unlocked the unlock experimental area Panel/Last Tab.
So I would advise anyone trying to downgrade their router to v6.1.4.3 to first delete or rename their DMTv7.35.ini file before experimenting with the firmware.