Are there any extensions, open-source Magento modules, or tutorials that make adding a step to the one page checkout any easier? I've poked at the core code for this a few times and it seems like there's a lot of hard coded step information.
Magento 1 – Adding a Step to the One Page Checkout
checkoutmagento-1onepage-checkout
Best Answer
The short answer to this question ended up being "No". From a high level, adding a new checkout step meant
Adding a new Block to the one step checkout page
Having that block register itself as a step
Connect that block with a prototype object that handled all interactions
Rewriting several core Magento methods to insert the block as a step
Some ajax trickery to get the progress stuff working.
Covering this in total is beyond the scope of a single Stack Overflow question. The tutorial linked in another question has a lot of what you'll need, and I also created a new commercial extension to make all this easier.