Here is a new picture, with VGA colors fixed. I still use a 2-bits R-2R ladder for output, so the colors are still a bit different from what's produced by the original adapter, but I can't say that the result is better or worse, so I'll probably keep it this way.
On the left, the original adapter's output on a LCD TV, on the right, VGA output of my adapter.



The image isn't very crisp (I get a better image using Starter Board's own VGA output, even if it only features 1 bit per component). It means that it's now the time to make a proper board, instead of having such a mess of wires hanging around ;)
The desing still uses less than 60 macrocells, so it could fit into a XC9572XL for instance, which is probably what I'm gonna use at the end.