I use a vga -> TV adapter with a radeon, and I use custome modelines.
They work with Xorg with the open source radeon module, but not fglrx,
because fglrx doesn't reconise the composite option.
It seems radeonfb have the same problem, because modes I define in
/etc/fbset.conf (or fbdev.conf I don't remember) doesn't work.
I have seen a patch on another website but it's for an old kernel and it's
two years old. I wonder why it wasn't incorporated upstream.