Hi,
I have such box at home:
http://www.zoobab.com/belgacom-tv-scientific-atlanta
Any idea where I can find the sources of the bootloader, and how to flash it with JTAG?
Interesting...according to the UART log it uses "Focus Boot" just like my smp8634 target here http://www.t-hack.com/forum/index.php?topic=859.0
Most important to find out if your bootloader is signed with the standard SDK keys or using a 3rd party vendor key. Follow the above post to find all the info to dump the bootloader and XENV via jtag.
The modchip is only for certain versions of Microsoft's WinCE bootloader. The Atmega firmware needs to be updated for the "focus boot" bootloader. Then we can run unsigned code.