Can a 2009 MacBook Pro use more memory than Apple's 'maximum RAM' figure of 8 GB?
As bmike refers to, YMMV, but...

Yes.  I had a 2008 MBP which has a listed 'maximum' of 4GB even though the slots would accept up to 8GB.  This machine worked wonderfully for a couple of years.  It was replaced though still works.

I am not sure if installing above the supported maximum voids your warranty, but it may be something you want to look into.  I just didn't care when I did it.