If the D400 is a truly excellent breather, then I think that my emergency bottle is right where I want to put it.
This doesn't make a lot of sense, but it won't hurt anything. To me the ideal pony reg set up would be a MK2 with an old metal case 109 2nd stage. MK2 because it's an unbalanced downstream piston that is dirt simple and almost incapable of not supplying air. There are no dynamic o-rings subject to HP air, which means a HP leak is almost unheard of, and coupled with an unbalanced 2nd stage, will give low-on-air divers some warning before the tank empties. I have read, (but not tested for obvious reasons) that the MK2 is more flood-tolerant than diaphragm or balanced piston 1st stages as well, and it makes sense given the design.
The 109 is perfect as a pony 2nd stage because it's extremely durable and can take a serious beating, has an adjustment knob which allows divers to really crank it down when not in use, preventing an inadvertent free flow. It's a very smooth breathing regulator as well, can go years between rebuilds, and service parts consist of 2 o-rings and a puck seat easily available for pennies.
But your MK10/D400 will be more than sufficient.
My cave set up consists of 2 MK10s, a D300 primary and a 109 alternate. I'm thinking of switching the MK10s out for MK15s, and may do that someday when I'm really bored, there's no decent sports on TV, and I'm trying to avoid doing something productive.