Let me get this straight... you have items in MC that have no metadata (other than the required filename) and you have corresponding items in Excel that have all kinds of metadata and you want to get the Excel metadata into MC?
I don't know about Couch's case, but I have a similar case where I have lots of items in MC that already have metadata (tags), but which I want to mass edit in an external editor more felicitous than MC. For example, one that can do find & replace with regular expressions, something MC doesn't support. Then I want to import that externally-edited data back into MC to update the metadata. In the past, I've been told this is not possible. If you have a solution, I'd love to hear about it.
As for Alex B's MPL solution, I've tried doing that (exporting to MPL, editing the mpl, importing), and either I don't understand it, or it doesn't work. When I do Tools | Import Media and point to a directory with an mpl file in it (with mpl being one of the checked import types), it tells me no new files found. Is there some other import mechanism I don't know about?