I'm not sure if you noticed that at below URL

http://www.microsoft.com/windows/ie/searchguide/en-en/default.mspx

where generally you can add some custom predefined search providers for IE7 you can also create your own custom search provider.

Quick test with Polish news&media portal called Onet.eu resolved in below Xml which is indeed search provider definition:

 <?xml version="1.0" encoding="UTF-8" ?>
<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
  <ShortName>Onet.EU</ShortName>
  <Description>Onet.EU provider</Description>
  <InputEncoding>UTF-8</InputEncoding>
  <Url type="text/html" template="http://szukaj.onet.pl/query.html?qt={searchTerms}" />
</OpenSearchDescription>

Simple, huh? :)