I bet that has nothing to do with the streaming engine - it's most of all a GUI challenge - how to integrate more kitpieces without compromising the current interface? There are several solutions. One of them is multiple tabs: keep the current 12 slots, but enable switching between multiple sets of 12 slots, each of them being available at the same time, each could have its own output (as all current kitpiece slots can have as well). Just switching the tab enables you to edit 12 new slots. Clicking on a slot automatically scrolls to that channel strip in the mixer. Easy as pie.
Take a look at these images. Since slot numbers do not comform to any particular kitpiece type, you should be able to drag and move the mixer channel strips to move them at your will, and sort them appropriately. The overhead, room, bus and master outputs would always be available at the press of "MAST" button, or when you scroll to the end of the mixer with left-right buttons (which are on bottom right, below MAST button).
Adding a scrollbar to this (which would respond to mousewheel, of course) would make things even faster.
Now feast your eyes on these images. It sure is easier to have it this way, don't you think? BTW, I lost about 6 hours for these
Slots 25-32 (last section, plus Master)
This happens when you click on MAST