Not ideal to be honest, although I did use this approach for a week or two. You will rely upon the LJ commenting feature, and you will have to create a suitable LJ style to make it work.

Interestingly, you may not be aware that you can, in fact, have as many styles are you wish. 'They' will not tell you this anywhere... but you can. ;)

You could have one style for people who view your journal directly at livejournal.com, and another for your importing IFRAME.

You simply reference them (in an IFRAME) like this:

<!--        Setup journal		-->
<iframe src="http://www.livejournal.com/customview.cgi?
user=rowanboy&styleid=186838" width="100%" height=300" frameborder="0" scrolling="auto" 
allowtransparency="false">
<!-- Alternate content for non-supporting browsers -->
          Your browser won't work here. You'll need IE5.5+
</iframe>

When you define your custom style at LJ, you'll be told the style id to use. Note that this will only work for paid users of LJ.