I searched a bit for this, and found a few threads of the same question, but with no definite answer.
First one is simply "use feeds":
Second is "try to integrate with PDFSharp"
And last is about connecting Apose with XMP
None of these have any definite answer as to how you generate a PDF combined with the awesome power of XMP.
So here's the deal :)
If you know of a great PDF generation library (preferably free) for ASP.Net, please add that as an answer with a link on how to get it.
If you have an idea about how to combine one of those libraries with XMP (Feed or other), reply to one of those answers.
If our dear Kelly thinks this is a topic great enough for its own expansion part, and someone adds a particular solution which sounds really appealing, we can hope it will be added in a future update! :D