setting rel=nofollow on a node-by-node basis

I'd like to set the 'rel' attribute on a node-by-node basis. I realize this isn't the defined functionality of this module, so I'm asking for advice on how to go about it.

I've been thinking about simply adding another CCK field to my content type, either a 'rel' text string or simply a checkbox to enable/disable the Link module's default 'rel' attribute. Given the value of that other CCK field, I'd like to modify the attributes of the Link field, similar to how the Link field already removes the 'rel' attribute for internal links.

That led me to wonder whether I could trick the Link module into thinking that a link was internal (even though it wasn't) to allow the current/default functionality strip off the rel="nofollow" attribute. But how could I do that? I don't know.

Alternatively, is there a clean way to use hook_link_alter or some other hook to modify the CKK Link field? That would be nice to use a hook so I'm not modifying this module's source. I'd need to be able to view the other content of the node to make the decision of whether or not to alter the CKK Link field, and hook_link_alter doesn't (by default) seem to give that to me.

Any advice on how/where to best do this? Thanks.