Logical replication - Manually triggering update of data on destination tables (after schema change)
You would want to add the new column to the subscriber with the appropriate default already in place.  You will have to fish that appropriate value out of the publisher, I don't know of a better way to get it.  After the column is added, you might want to either drop the default, or turn it back to now().  If the only way the table gets inserted/updated is from the publication, then this isn't necessary as the publisher always sends the actual value and so doesn't use the default.  But leaving the unused default in place seems like a recipe for confusion.

begin;
alter table pgbench_accounts add column newcol2 timestamp not null default '2019-03-26 09:44:56.391128';
alter table pgbench_accounts alter newcol2 set default now();
commit;


Note that in version 10, this will still be pretty resource intensive, as adding a column with a non-NULL default requires a complete table re-write the subscriber, just as it did on the publisher. (This was changed in version 11 to use "fast default"). But the intensive work is confined within each server, not the connection between them.