I am finding that placing quality comments on popular nofollow blogs allows for a potential dofollow backlink. Sometimes the blog and comments are copied to a different website that does not use the nofollow. There is also the possibility that the webmaster who maintains the blog will remove the nofollow tag attatched to your link because he trusts that you will not link-spam.
I don't know if the blog software has an easy way to remove the nofollow tag. Sounds like once you know how to do it, it's simple. After watching youtube videos on this subject, I know that some webmasters claim to have removen the tag from their blog's comments. Matt Cutts, head of Google's Webmaster Spam Team, also says that it is possible to create a special meaning for the nofollow tag through him, in order to apply it to any one web site.
i'm not sure what is available but i have seen pages before where some comments were dofollow and some were nofollow. not sure how it is done
If you are only placing links for "SEO" then you are doing it wrong. Links should generate traffic in and of themselves, SEO is a side benefit. If you get direct targeted traffic from your link campains the SEO will follow. Google measures a whole lot of factors in determining rank.