The problem you've got here is that to meet your space requirement you will need a back pack. If it's a back pack, having the hood on ready to go is going to be of little use when you have to get the bag off your back, get into it, remove camera, put back pack on, remove lens cap and then get ready to shoot. If your subject is wildlife, there's a good chance it will be long gone with the sort of bag you are looking for.
The quickest, accessible, type of bag you will find will be something like a Lowepro Slingshot. Having an accessible bag, but needing to reverse the lens hood will be substantially quicker in practice than any backpack, especially the big ones you will end up with if you want to fit camera+extender+lens+hood.
However, again if wildlife is what you are after, then really the only way to operate is to have the camera out of any bag. The noise of velcro or zips will ruin any surprise you may have anyway.
So again, really you need a bag that will carry the camera to wherever you are going, that isn't huge or difficult to get equipment out of and then perhaps invest in a better strap to carry the chosen lens+camera.
This is speaking from experience using a 7D + 100-400 L for wandering about taking wildlife photos which is very similar size wise to your setup. I use a Lowepro slingshot, and have the hood reversed with cap on, and can get the camera out v.quickly and be ready. However, ideally, carry the camera out of the bag if you need quick response. It is much quieter and pretty much how I operate.
If you don't need a really quick response, then again, seriously, reverse the hood, you'll be able to get a much more portable and easy to access bag, and reversing the hood takes seconds. If you need to grab a really quick shot, not having the hood on the right way won't mean no shot. Just remove the lens cap...