The Menucool Tooltip supports Ajax that will retrieve tooltip content from external files. Make it SEO friendly (forcing the tooltip content crawlable) is quite simple here. Just add a real link to the file as follows:
<a href="foo.txt" onclick="tooltip.ajax(this, 'foo.txt'); return false;">foo</a>
When the linked is clicked, the tooltip will be populated. The "return false" will cancel next href navigation to the foo.txt, but Google, Bing, ... will following the link and have the content crawled.
If your tooltip needs to be populated through the onmouseover event, you can do it this way:
<a href="foo.txt" onmouseover="tooltip.ajax(this, 'foo.txt');" onclick="return false;">foo</a>