Host specific parsing

Hi,

has anyone built a parsing plugin which decides on a per host basis how the 
content of the document should be parsed?

For example, if the title of a document is in the first <h1>-tag of a page for 
host1 , but the title for a document of host2 is in the third <h2>-tag, the 
plugin would extract the title differently depending on the host.

In my opinion something like a dispatcher plugin would be needed:

-          Identify host of a document

-          Read and cache instructions on how to get the information for that 
host (database or config file)

-          Execute host-specific plugin

Do you have any suggestions on how to implement such a scenario efficiently? 
Has anyone implemented something similiar and can point out possible 
performance issues or other critical issues to be considered?

Thanks in advance.

Kind regards,
Martina