There is one caveat to using this method. When the external xmp file is loaded, it does not clear all the variables when it loads. It does over-write all fields for which data are provided. Thus, you may not get what you expect. In order to assure that the only fields that are populated when the external file loads, you need to first open the template and clear it. (Or remember to do this before you close the template after using it.)
I build all of my external xmp files using exiftool during which I write only the fields for which I have metadata. I suspect (but have not tested) that there is a way in exiftools to write a command that will clear all the fields--perhaps by writing something like -xmp:IPTCfieldName="" which fils the field with a a blank. there might be something fancier.
This does open the possibility of having a standard xmp file that applies to every image--i.e., copyright info, contact info, etc. and then having a secondary xmp file that leaves these fields blank, but has stuff that is specific to an entire shoot. One could first load the most general info, then use the same drop and drag for the second file. I have not tried this, but will give it a shot. It has interesting applications if it works as suspected.
Bob