It's already possible to create overflow systems without the need to add any additional exits on machines. For the most part, adding exits would just make things unnecessary complicated. As it is, all that you really have to do is to keep putting your belt through a series of splitters until you have reached near the theorical maximum and then, at that last splitter, send the extra out. That's all it takes. Also, I would advise against the use of refeeders or complex balancing systems. They create more problems then they solve. Just keep things simple, run everything in series and it should work just fine.
The only real improvement that could be made here is to have the ability to designate the exit on that splitter as being overflow, so that it would only ever be used once the other lanes on that splitter have backed up.