How would one take this git flow model and add a third environment, staging, to sit between master and develop?

My initial thought would be to not change anything in the structure and to simply run a deployment script to export to the staging site using scp/ssh/rsync and ensure things are working properly before doing the same thing on production.

Just think of the release branches as staging branches--once something's ready to move from develop to staging, you make a new branch, so you can keep working on develop while you test staging.

Probably bears saying that you can install git-flow which takes care of a lot this for you. It's pretty neat. You might like it!

I've looked into and played around with git flow. I decided against it due to the abstraction. I'd much prefer if everybody on the team understands the underlying commands. I'll probably end up looking at the source to keep my bash-fu in check to see if there is anything the branching model article missed.

Same here. It's straightforward, just forward the link to everyone.