I see no reason why you couldn't put additives in the resivour. It will circulate as you mentioned.
I have not needed to use Water Wetter though I heard it is an excellent product. I added a couple ounces of coolent/water to the resivour and I am not ever coming close to an overflow. If your coolent level looks high, you can siphon some out of the resivour and replace with Water Wetter. You just need to find the fluid capacity of the cooling system and add the corect ratio of WW. As I mentioned, I have never used WW but I doubt you need more than a few ounces.
If you have a spill over, it will probabley happen when you shut the bike down. You could have a pan ready to place under the overflow hose. You don't want that stuff on the farings as I believe it's bad for paint.
'08 MIDNIGHT SAPPHIRE BLUE Now Deceased