Sorry about resurrecting a dead thread. However, since I kept on finding this post everytime I searched for "n900" and "myvu", I"m just going to leave one potential solution here.

I managed to get the n900 to automatically detect the myvu's presence by putting a 82ohm resistor across the ground and signal. There's some talk about using a different value here: http://talk.maemo.org/showthread.php?t=45408 For me, the detection only seems to happen when I plug the tv-out cable into the n900. So performing the connections in reverse order fails.

I have a suspicion that the n900 autodetects the isolating transformer that the tv uses for the incoming signal. Don't quote me on that though.

Regarding the OP - I believe you're in for a pretty interesting ride if you want to stretch the borders I'm not even sure how to do it, however, I think the place to start looking is not the part that drives the tv-out signal but figuring out how the n900's framebuffer is setup.