Unfortunately I can't speak for how SEOmoz handles rewrites like this if it's already crawled the page.

The rewrite rule you're using looks like it's only rewriting the www portion of the URL, not index.html. So alone it wouldn't do anything to solve dupe content issues. (someone please correct me if I'm misreading the rewrite rule)

Here's a link to what I used to write a redirect for index.html on another site.

http://www.webmasterworld.com/forum92/6375.htm

I think it is a fairly safe assumption to make that SEOmoz is smart enough to realize if you're got a redirect in there (providing that its working). I'd still recommend taking a look to see if Google has cached or indexed an index.html version, though.

Edit: my personal, highly technical, acid-test for an index.html redirect is just going there and manually entering the url with index.html on the end, rather than waiting for a recrawl to see if you're heading in the right direction.

As you did, I'd normally handle this with a 301 from index.html to the root domain. When you say that it's "not had an impact" do you mean that the SEOmoz dashboard continues to show an error after it re-crawls, or that the search engines are not picking up the redirect?

SEOmoz dashboard does a great job, but I'd check to see how the search engines are actually indexing yourdomain.com/index.html vs. yourdomain.com also. If the search engines are indexing it as you want them to, then I'd be inclined to ignore the dashboard error.

I apologize if this is a stupid question, but I assume you manually checked that the redirect worked?