Altering same table across multiple databases
I think the fundamental question you need to ask yourself is how much of a guarantee you need that this has propagated to all db's.  Do you need every change to commit or roll back together?  If so you should probably write a script in a programming language of your choice (Perl, Python, or the like), send the DO block there, and then use PREPARE TRANSACTION on every db connection (you'd open up one connection per db).  If all of these are successful you commit.  If any one fails, you roll back.

This is actually one very helpful application of PostgreSQL's transactional ddl in that you can guarantee that all your databases have schemas which are kept in sync.