I always like these kind of postings. Proofing that some things are possible :) In his example he adds a WebPart Page to a Document Library by altering some code in the well-known ONET.XML. Do not be afraid of editing this file. Its quite easy to understand whats going on.
Check your documentation "Microsoft SharePoint Products and Technologies 2003 Software Development Kit" (file spptsdk.chm) and look for ONET.XML. You will find an article describing each bit of this file.
The article describing pre-populating a document library can be found at:
http://blogs.msdn.com/cjwalker/archive/2005/10/26/484879.aspx
Thanks to Christopher Walker!