I think Jukka already worked on something really similar to the ExtParser in Tika. See: http://tika.apache.org/0.7/api/org/apache/tika/parser/ExternalParser.html
yes, that's the one I had in mind
One of the plugins which hasn't been ported yet is the feed parser. We could rely on the one we recently added to Tika, knowing that there is a substantial difference in the sense that the Tika feed parser generates a simple XHTML representation of the document where the feeds are simply represented as anchors whereas the Nutch version created new documents for each feed.
There is also the parse-rss plugin in Nutch which is quite similar - what's the difference with the feed one again? Since the Tika parser would handle all sorts of feed formats why not simply rely on it?