Can't do it off-the-shelf, but it's quite easy to write a custom field that appears on the issue navigator and shows the comments. I think there's several in various plugins at https://plugins.atlassian.com/ and I think you can do it with a message field from the Jira Toolkit plugin (not tried it though, so don't quote me)
However, I'd warn you against it - the comments are an open-ended field and if you include them all, you can completely kill your view. I've got issues here with 30+ comments, and other issues that have short essays written as comments, and as soon as a filter reutrns one of them, the issue navigator becomes stupidly large and unweildy.
A better implementation is one that shows just the last comment.
Edit - Bother, I didn't notice the OnDemand tag, sorry. I'm not sure you'll be able to implement anything in there, as plugins are restricted
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