Sounds like you could do that using SPD workflow.

Poster is saying they want to write to the file  itself...unless you
have an amazing custom activity that you would like to share (O; SPD is not the
answer. This will require code.

Try using Event Handlers......................

You can assign the file  properties using sharepoint  listItem. After getting
the listItem using SPListItem, try to do like below:
listItem["Author"] = "Jackson";listItem["Subject"] = "Chemistry";
after assigning all fields  finally update the listItem and the changes will
appear.
listItem.Update();

This is writing to the list. Poster wants writing to the file  stream
itself. If you right-click on a file on your system and choose properties  you
should see a tab for Summary info. Poster wants to add data  there.

Check out this link. I know it’s a subscription service but you should be able
to scroll all the way to the bottom and find more information. It seems there is
a .dll on MSDN somewhere...