Ran my unit like so:
Butt-Mounted Bailout Bottle: Plugged into Whitey Valve for Dil Switching, Connected to 2nd Stage Reg clipped off on my D-Ring (later put on a "necklace" a la DIR dudes), Hooked into BCD Inflator.
This left my onboard Dil sphere for Semi-Closed and Makeup Gas for the loop.
I would run only Air in the Bailout Bottle, and keep the Mix in the onboard sphere. (Logic: Even on deep dives, you can generally figure your 1st Stop will be around 1/2 of your depth, which makes breathing Air ok, so why bother with mixes of different sorts, etc? - If you're in a Bailout situation, you're not hanging around your MOD anyway (unless you're in a Cave or Wreck - different story - I'm an open ocean dude, which is how this philosophy evolved)).
If we were doing really deep or long dives, we'd carry additional gas, which had identical quick-connects on the LP side so we could plug that into either the OC 2nd stage (the hose to it was broken in the middle with a Quick Connect, so you could keep it in your mouth while you plugged into another gas source).
We would also stage bottles on the Anchor Line (Bottom, Middle, and O2 at 20 feet) - and sometimes we'd even stage bottles around the Reef we were working on - making the aforementioned butt-mounted bailout bottle a form of "travel gas" to get to the next stage.
We were taught the "total reliance on the unit" method of diving back then, so the idea of using the onboard dil for anything other than semi-closed on the unit was considered a no-no...
Disclaimer: Just discussing what we did - I know the methods of Bailout have evolved, but I still like the idea of having your Bailout Gas plumbed into your unit somehow so accessing it is easy...
Kevin.