I have a similar question - I'm trying to do the same thing, but I'd like to have the value I type into the BOM appear on the referenced part's datacard.  Currently I have the version-free variable on the datacard and the BOM displaying it, but if you change the value in the BOM, it doesn't update on the part's datacard.  The values can be out of sync, which is obviously a problem.  Is there a way for them to stay linked?  I've tried several of the options for configuring the BOM columns ('Look for variable in referenced configuration'), but it doesn't seem to work.