The softbox should be white. It won't give the colour from the gel a cast, and so you'll basically get the same effect as with the bare flash and a gel, just softer. You hit it on the nose with the saturation bit though. If you want more saturated colour from a gelled flash, the key is to actually turn-down the flash output; fire enough light through any gel, and it'll turn pure white. (This can be used to your advantage for creative lighting of a background, say, by putting the hot spot directly behind the subject out of the camera's view, and creating a halo around the subject of colour slowly increasing in saturation outwards; tricksy, but doable.)
That's what I was trying to do but I couldn't get it! I did have the flash power high as that's what I was thinking I needed it to be at, but your expalnation of a lower flash power makes perfect sense. And since it's a white muslin, wouldn't you need a three strobe set up? One behind, two in front?
I tried lighting the front with continuous lights as I only have one strobe, that didn't work as it would drown at the background color. Hmmm.. I want to get this shot, any ideas?