I like this idea. I though about something like that when I had to modify my setup to have some iron ore for steel production. That would be a great implementation. Maybe allow upgraded belt to do retroactive speed? Not sure that is how I should say it like this but i.e. one MK3 belt could also be setup to have MK2 or MK1 speeds.
What I though about on my side was to have splitter be able to program to have the split i.e 2:2:1 or 3:1:2.
So one splitter could feed let's say 2 iron ore to my first smelter, then 2 iron ore to my second smelter and 3 iron ore to my foundry before starting the cycle again.
Not sure this may be included with smart splitters. I have yet to unlock them.