If you can pull the XML feed before the Title gets set then you could easily use information from the XML feed (and thus the H1) in the title.
I guess you need to ask yourself these questions:
-
Can I pull the XML before the title is processed?
-
When creating the H1 are there any other dependencies that are only available after the Title is created preventing you from doing this before header.php
If you can then you just need to add your code and some conditional statements to the Title tag within header.php