Upgrade databases from SQL 2000 to SQL 2012

I understand that you can't just go from SQL 2000 to 2012, you must first attach the db's to SQL 2005 or 2008.

Not all of our clients have access to an SQL 2005/2008 nor do they really want me to install the eval 2008 onto their server

So what I want to know is what really happens to the db's when you attach them to a later version of SQL?

If it just changes some header info or something surely we can make a script to 'trick' the db's into telling SQL 2012 that they've already been upgraded to SQL2008, or a script that actually does upgrade them?

Thanks, I hope that all makes sense.