Sorry, have found the answer I was looking for.
Sorry, have deleted your answer by mistake. I think I've read that specific boxgrinder version did not support building centos on eg. fedora, but then I have found an answer in boxgrinder FAQ section saying that it's actually possible.
Great! Just for anyone else picking up this thread, you *can indeed build Centos 6 with BoxGrinder*.
kaos monk; if you have found any documentation that says we can't build centos 6, please let me know where you found it and I'll update it.
edit: repost after kaos monk accidentally deleted the replies
You definitely can build centos 6, even on fedora 15, which is great!