I used 100% green antifreeze and pulled the manifolds off the pipes and removed the gaskets. Put them in the crock pot on high, with the lid on. Left them all day. They came out clean and the antifreeze was filthy. I had a big pot so I did like 6 pipes at once. There were a few really dirty spots that I had to simply rub off the loosened remaining residue with a rag. It probably would have cleaned that off if I left them in longer.
No harm to the pipes, coating, etc. It does take awhile, but if you have a day or so of maintenance to do anyway, then whats the hurry?