I've been reading a bit about the Coldbox router... curious if anyone has experience adding it to a large existing project and how difficult it was? Ideally all of our current URLs would continue to work as is, and then we could slowly move pages/sections of the site into the router to have nicer URLs. Is that feasible?

Totally

That example is using an entire ColdBox MVC app next to the legacy site. Is there a demo/example of just using the router? (I read a couple blog posts from last year about it, but am still struggling to wrap my head around how to do it)

You can go another route without needing the router at first (but you can use it). I used the technique put forth by @lmajano in the ePub that I bought awhile back called “102 Coldbox HMVC Quick Tips & Tricks”. The VERY LAST item is the one I needed and shows how to put CB in place with an existing site and have them work side-by-side. I got that working and haven’t looked back since. Well worth the cost. https://www.ortussolutions.com/learn/books/102-tips-tricks

You can omit all the folders of the mvc app if you want

Sounds good, thanks! I had my boss get the book for me, so I'll give it a read 🙂