The purpose of these is to allow for maintenance/support people to do "things" with the device, they are not targeted at end-users. Might sound surprising but most all electronic devices have such "secret combos". Right now i am thinking of examples i have seen with TVs and HP ink-jet printers. In the printer's case it was more than the "press and hold these two keys to get demo page printed" - there was e.g. combo to make it accept cartridges that were being rejected for having failing ink cartridge chip timestamp check (an example of counter ink-refill measure).
So anyway, my point is if indeed dev.mode access has been disabled on purpose - as
news said - there won't be way to go around that from other secret screens. And to download different firmware than the general population, that "honor" has to be specifically bestowed upon your device S/N by someone at the (Sky?) Co side, it's not as easy as switching servers on your side.