Someone else might have a better idea, but I think as long as you're not trying to make money off it creating a roster/warband management programme should be fine. Other people have made roster apps without any trouble I know of (which might actually save you some work), I'll see about finding a link...). GW aren't currently paying much attention to tabletop Mordheim, so it shouldn't be an issue -- again, as long as you aren't trying to make money off it.