By default, the HTML parser of OpenSearchServer try to extract only the visible information of the Web page.
It is possible to retrieve information stored in meta only if they use a specific syntax. Your meta should be in the form:
<meta name="opensearchserver.field.searchtype" content="contruction_collection" />
You can also populate several fields:
<meta name="opensearchserver.field.searchtype.anotherfield" content="contruction_collection" />