In DNG v6.0.2, this would be easy....export artifacts into a .csv (while showing the new attribute column), enter in your values in the resulting spreadsheet, and re-import to update each artifact with the value.  Unfortunately, v5.0.2 does not have round-trip import/export functionality.

However, you should be able to update the attribute in bulk, although 200 is the most you can do at one time using this method. Select the first artifact you want to update (check the box), scroll down to about 200 artifacts, and shift-click the last artifact to be updated.  The bottom of the window should tell you how many you have selected...make sure this number is 200 or less.  With all the artifacts selected, click on the pencil icon for any one of them and select "Edit the Attributes for # artifacts..."  The window that pops up should list your new attribute which you can edit and hit ok.  Prepare to wait while DNG updates each one...if you have thousands of artifacts, this is going to take a while, but I don't know of any other option other than upgrading to v6.

Good luck!