I just noticed a problem with log_statement = 'ddl' and log_line_prefix containing '%x'. If the statement is the first in the transaction, it will be logged before it is executed, and more importantly, before a transaction ID is assigned. That means that %x will be 0.
If the administrator has configured postgres like this in order to ease PITR on bad DDL, it's actually of no use whatsoever and they'll have to use pg_waldump to try to figure things out.
PFA a simple patch that I hope addresses the issue in a clean way. It also handles the same problem for 'mod'.
I'm undecided whether this is a bugfix or an improvement. I'm leaning towards bugfix.
FWIW, I'm with Andres on the general points -- especially about having to look at overheads etc...
For your specific usecase, and until the actual logging can be rethought on it, I suggest you take a look at event triggers for it. E.g.: