If you use just one disbtn, it'll look kind of ugly. What picture would you use? That one picture would have to serve as the disabled button for every icon you use, which is pretty limiting. DISBTN icons should only be around 6 kb each anyway. You can reduce the quality without anyone noticing since (1) it isn't shown all the time (2) it is hard to notice since everything will be darkened.
If you really don't care about having a disbtn for every icon, then it is possible to use one disbtn and import it under every path for the other icons (you'll just change the path. so for each icon, you'll import that disbtn and change its path to match it). That'll still take up a lot of space, but if you make it completely black and choose paletted compression, it'll hardly take up any space at all, even if you import it thousands of times. But I don't recommend it, that would be weird to the gamer. Almost as weird as it turning green. But that is an option.
In general, I recommend that you compress your icons (especially the disbtn's) instead of trying to consolidate it all. Use BLP Lab. You can set the disbtn quality to around 40% and save a few kb. That way, they won't really take up much space at all. Even if your btn + disbtn take an average of 20 kb, that is still enough room for 409 icons before you hit the 8 MB limit.