Unfortunately I don't have anything that isn't immediately searchable via Google (for example: http://disconnected.systems/blog/nim-on-adruino/). I was indeed planning to write a chapter on this but due to page count constraints I wasn't able to, that said I will do my best to write an article on this at least and post it on nim-lang.org as well as on book.picheta.me :)

I do also know one person that was working on some cool microcontroller stuff in Nim, I keep asking him to write a blog article about this but he's currently busy with other things. Maybe soon it will happen to though. If so I will encourage him to post it on Nim's website and if he doesn't I will definitely tweet about it here: https://twitter.com/nim_lang.

Sorry I can't help more!