So it wouldn't work if I set the U to 2x in ZBrush? - It's got a setting for that but I don't know if it would work the same.

But it's actually quite easy to load a Roxie into ZBrush and start painting on her, I've been using a techinque Shane pointed out a while back, but it would definitely be much better if your trick worked in it.  ZBrush works differently to other programs, the idea is that it subdivides the model into as many polygons as you have pixels on a texture map, and you paint directly onto the polygons at whatever resolution you like.  When you've done that, you transfer the painted polygons onto the texture map so that the texture map can be saved and used in Poser etc, so it can be done quite easily, it's just different.

Still, I wish it would load-up each map simulatneously.  You have to load the head map, transfer it to the polypaint, and then do the same for the body map before you can work on it, thats the only thing that annoys me about it.