Just some thoughts.
a. There are no actual image files or copies of them actually in the Catalog. The files are only "referenced" in the catalog.
b. Will you have actual files or copies of the originals stored in different locations / drives / folders.
c. If you have one original file, imported into multiple catalogs and you write metadata automatically to the file that will be done. However if you then open another catalog that has the same file the changes will not be applied automatically. You should get a warning that there is a change with an option read the info from the file.
Working with one Catalog and using Collections and Smart Collections you should be able to achieve your goal. Even without the "Automatically write changes to xmp"
Agreeing with DdeGannes who said
Working with one Catalog and using Collections and Smart Collections you should be able to achieve your goal.
and adding to this point, working with one catalog is a lot less work than what you are trying to do, and lets you accomplish everything you stated you wanted.
Echoing both answers, you are really making a lot of work for yourself, and how will you ever know which catalogue has the right version of the image?
Just think about the weaknesses of all catalogues saving xmp automatically. As a simple example, image 123 might be in just two catalogues and let's say you know it was up to date in catalogue 2015. Going to photo 123 in catalogue Weddings, imagine if you forget to do a Read Metadata on that picture and then, maybe accidentally, you apply a star rating to it. LR now goes and saves all its adjustments - older adjustments - and metadata back to xmp. You're going to have to keep your head very tightly screwed on to keep track of this, and will spend a lot of time jumping around from catalogue to catalogue. You will also lose some of your work (xmp doesn't record flags, stacking, virtual copies, history steps...). So for one thing you specifically mentioned, flagging, you can't sync it across multiple collections.
Most workflows like you seem to envisage can be satisfied by learning to use the File > Export as Catalog and File > Import from Another Catalog roundtrip. When I need a wedding's photos on another computer (eg my laptop for viewing with the client), I export these pictures from my main catalogue to a mini catalogue via Export as Catalog, make any changes on the laptop, return them to the main catalogue by Import from Another Catalog, then dump the mini catalogue. Everything except published services is included in this method - including flagging. It sounds like you should be looking in this direction rather than needlessly fragmenting your workflow across multiple catalogues.
thanks for the various answers. I've considered most of what was written already, but the gem in all this that I was looking for was from John's answer:
You will also lose some of your work (xmp doesn't record flags, stacking, virtual copies, history steps...). So for one thing you specifically mentioned, flagging, you can't sync it across multiple collections
This is consistent with what I've experienced - I wanted to find out if there was a way to not loose some of my work. The goal was to find a way that everything would sync automatically & properly so that I would always access the identical image from various catalogs. This is what I thought XMP would do - but I experienced that it didn't and now I understand that it won't do it properly, regardless of the settings. Flags were exactly the piece of information I was missing when I tried this.
I am surprised that everyone is telling me to use one Catalog. I am sorry but I don't believe that it will scale - because that is not what I have experienced. When my LR4 catalog got to about 50,000 images, it slowed down dramatically. Has something big changed in LR5? I have read many threads about this, most seem to say that already in LR4 it shouldn't have slowed down, but it did. No, my hard drive wasn't full. I just shoot a lot, mostly with a D800.
Some people in this forum report catalogs of 250,000 images or 500,000 images. Catalogs are designed to handle this, and if it slowed down, there are dozens of possible reasons, the last of which on the list of possible reasons, with likelihood of less than 0.00001%, is that you had 50,000 images in your catalog.
Catalogs are databases with references to files which are held separetely on the file system. A database with just 50,000 records is tiny; even a database with 500,000 is nowadays small, 20 years ago it might have been huge.
The performance problems are not related to the size of the database, they are related to the camera makers pushing the size of the sensors and computer manufacturers pushing the size of the displays. There is some talk on this forum about Apple's new 5K displays and to be honest I don't think many people have bothered to think about what combining LR, a 5K display and a Nikon D800 will do to performance.
Forget about the D800, there are contributors to this forum that shoot with a Phase One digital back which can have sensors of 80 megapixels or more.
The bigger the sensors and the bigger the displays, the more important is to get things right on the camera.