A model could be Stackoverflow.com which has an option to mark a post, thread or single comment as "karma neutral" (in LW terms; they call it something else). The up- and downvotes still work, they're just not applied to the author's karma.
There's some slight potential for abuse if that were transposed here, insofar as someone could take advantage of it to troll with impunity; but I don't see someone being amicable enough in the first place to amass much karma, and then turning around and becoming a troll.
I'm personally uncomfortable with endorsing another voting system that functions just like karma but with a different name. Having more than one set of numbers that can fluctuate from positive to negative and are differentiated in name alone sounds like a headache, as well as an interface that would scare me away were I not already familiar with the site.
Instead, I propose simple "I Agree" and "I Disagree" buttons. Rather than just tallying the amount of votes for either, it should simply attach one's name to the post via a pulldown tab...
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