How often do you expect to make the schema changes? I mean I quoted this bit "...make schema changes sometimes on a daily basis" – is this realistic, or a kind of business insanity typically caused by bad management? Ditto "...but continual, recurring downtime due to schema changes". This really looks like a failure of management rather than a technical problem to be solved.

Also aren't you likely to be doing something larger than just a schema change very often, in which case that would necessitate replacing your application, so changes are not just restricted to the database. You now have a bigger problem of co-ordinating app and DB changes.

I also asked to do you need permanent uptime because in a lot of systems, especially smaller ones (and by the long tail most systems are going to be smallish) the users are very tolerant of an hours' downtime a month, for example.

"Once you have beyond a few million rows in a normal RDBMS, schema changes can take minutes to hours depending on the type"

That's a pretty strong claim; what kind of thing is going to take hours that your database can do consistently? Does it even take hours? I had a 100,000,000 row table of unique ints lying around so I put a foreign key from itself to itself (a bit daft, but just for timing purposes. DB is MS SQL, table is fully hot in memory)

alter table [tmp_ints_clustered] add constraint ffffkkkkk foreign key (x) references [tmp_ints_clustered](x);

21 seconds.

What you're doing (if you can get it correct! Which I have to wonder at) is doubtless excellent for some very large companies, but in general... I'm afraid I'm not so sure.

Edit: I feel I'm perhaps missing your bigger picture.