I've just been updgraded to Sky+ and I've got an old Sky box (Thomson
286_544 aka Thomson
DSI4212C). How would I go about installing NetBSD (compatible with nearly everything) on it?
Has anyone ever done this?
I have no a "how-to" answer for this, I have very little experience with embedded devices and I'm pretty sure asking on linux or netbsd development lists probably would be a better choice.
If there's no an already running project which provides installable images this is going to be very tricky and risky... :-)
Usually the first approach is to open it up and find a "serial console" or JTAG points, it's something from where you could start working and see how the boot process is done. You probably will find out the bootloader software is running on your device and try to find information about it on Internet.
Bootloader step uses to be very important for extracting information about the device, its partitions, etc...also, it might have a tftp server (if the device has network) for recovery purposes and you might see its configuration.
Maybe, the easiest approach could be getting and official firmware upgrade (the device is probably already running Linux), try to modify it and flash it to the device. But again, this is risky and better have a way to restore the device (tftp, jtag, etc), that's why is so important to gather as much info as you can before doing anything.
Once familiarized with the flashing process and the device components, storage layout, etc...you could try to build your own image. For building an image, might be better to start off an already specialized project like OpenWRT, they have a working build system in a way you'll only have to worry about writing device-specific makefiles and so...
I understand this is not what you are looking for but without an already working project, you'll have to do everything.