As I just said, you add a second volume pot which runs your clean channel separate from the Volume pot that affects the Lead channel. I'd have to dig around and find schematics to figure out of it would be the same, but it's a known mod on the IIC+ if you search "Volume 1.1 Mod" or "Pettrucci mod".
For example here, the added a pot where the footswitch jack used to be which would act as "Volume" when in clean. Then the former "volume" pot only runs when in lead mode.
Off the top of my head, you'd maybe need to use a couple relays/LDRs to swap between them that you splice into the normal lead circuit switching?