1. I agree that once it's out there, it's out there. Nothing can be done about that.
2. For purposes of better conversion (getting people to the link from your main page) - Local Listings link should probably be on the main page and not just in the site map. That's one less click. "Where to buy Beachcomber Hot Tubs"
3. For purposes of better search engine visibility, you can create several regional pages if you have a lot of outlets and list the ones for each region. That would make it a tiny bit harder for competition to find the outlets, but it won't stop them.
4. Another alternative is to create a Web log and then put articles in it that link to each of those store sites. The Web log is useful for publicizing other aspects of your site as well. Since it has an RSS feed, search engines should pick up the listings pretty quickly.
5. You raise the question however of whether a search engine or Google in particular will list an orphan page (one with no links to it from anywhere on the site. My experience is that somehow or other they do, even when you don't want them to do so, though it may take a long time. That seems to be true regardless of listing in XML sitemaps.

6. Don't know if you tried this - submit the XML sitemap again (manually) to Google in Webmaster central and note if any errors are returned - they may flag the orphan pages. If your site map is huge, try breaking it up and submitting several. Google doesn't like sitemaps with over 100 entries or so - or so they say.

7. It is not a good idea to jump to conclusions from behavior of Google or other search engines in 1 instance. They are erratic and may do or not do things for reasons that are not related to anything we did or did not do. You can do the rain dance every day. Some days it rains, some days not. Depends on the whims of the great father in Google Central.

8. I am surprised there is not a regional Google Onebox local business listing placed at the top of such searches. Did you try registering each business in the google map? Or did Google give up on those?

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