I'm interested in a system where the entire operation of metadata import can be done from MC.
I don't think the benefits that might be had using this approach would come close to outweighing the data integrity issues I raised above. I wouldn't risk messing-up my database using it, even if it worked. And I'm sure it won't. It's already enough of a challenge controlling "the entire operation of metadata import" directly from PVD.
Yes, but one could still handle data integrity issues with a seamless system. You mentioned a new MC field "DateImportedFromPVD" which could be used to block import. Or some other mechanisms could be used.
It's not just that, though: Some users may have "lesser criteria" on data. For example, some of my friends use MC but complain about the lack of metadata possibilties for movies. Or my sister imports some movies into MC and her husband has no idea what the movies are about. A description, year, director might be nice for them.
But none of them are ready for PVD in its current state.
couldn't the plugin still extract all the data temporarily to XML and import that?
Yes, that's exactly what I am saying in the above posts. Use the mechanisms that are already there. (but pass the data through UDP instead)