Ben,
David's solution will work for you if the folder or file has an attribute or name that is consistent. For example, on your Project Summary page, you can have a field with a dynamic lookup, per David's notes, that will display an Excel "Bill of Materials" hyperlink. The SQL to produce this hyperlink would look within the collaboration folder for this project, for a particular filename (e.g. "Bill of Materials*") - perhaps including a MAX date in the event you are storing revisions (e.g "Bill of Materials Rev0," file date 1/1/2015; "Bill of Materials Rev1," file date 4/15/2015).
If your folders or files don't have a standard convention (e.g. everyone spells the same thing differently: "Bill of Materials," "BillofMatl," "BOM"), then this technique may not work for you. There's probably a variety of ways to do this, but a standard must be set for the SQL to follow (e.g. you could have different file names, as long as the file is always in a particular folder with a standard file name - "All projects will have a folder named "Bill of Materials" - then, the SQL could find the file in that folder with the last 'last modified date'; and hopefully, users don't put other doc types in that folder!).
If there is no standard, then you may want to look at just adding a URL attribute to your Project Summary page, where the user can copy/paste the URL address of your folder or document.
Short summary: If you have standards, use David's method - its really cool, works great (we're using it ourselves, for different topics). If you don't have filenaming/folder naming standards, use a URL attribute and tell users to copy/paste folder/document address.
David - comment if I've misstated anything, please.
Dale