M609 - as most crews on here know, doing an autoland on either a CatI approach OR on an 'unapproved' autoland runway for evaluation purposes is by no means unusual nor beyond the abilities of a capabale and trained crew. What IS contrary to procedures is to use the CATIII minima 'for real' as part of the approach in those situations (except, of course, in an emergency). There is, of course, nothing wrong with practising the Autoland.CATIII procedure 'call's' in the cockpit.
The crews you describe in your post should not use the words "CAT3 for practice" as that kind of gives the game away, really.
There is actually no need for any 'information' to ATC for an autoland UNLESS you are asking for LVP protections for the autoland for any reason (then you'll be popular!).