In my experience, all the library data for each file gets exported/imported regardless of whether or not it's saved in the files' tags. I say this because I've used this method to transfer the data for Audible files between systems, and MC is not able to save tag data to audible files -- i.e. the tag data for Audible files is only stored in the MC library while the files themselves remain unchanged. When I do an mpl export and import of Audible files, all the information gets correctly transferred over, so the mpl export obviously contains the library data. (Is that what you meant by "track data"?)
As to the question of whether or not this is a "genuine merge," this is not really a "merge" per se, but it's not a "hack" either, and the end result seems to me to be the same as what a "merge" would give you -- i.e. you end up with a library containing all the files from both libraries.
I'd suggest you simply backup your current libraries and try it -- it's not going to do anything to the files, it's simply going to give you a new library with all the files in it. If it doesn't work as you want it to, you can simply restore the previous libraries.
That said, I believe that the result will be what you are looking for. Once again, just make sure to backup the library, and don't make any changes to the tags after you try this procedure until you're sure it worked.
Larry