implicit cast of empty string to timestamp

Is there anything I can do to make postgres allow an implicit cast of an
empty string to a timestamp, so that a badly behaved application can do:

INSERT INTO SomeTable (timestampfield) VALUES ('')

Where timestampfield is of type typestamp. ?

From what I understand of the 'CREATE CAST' command, I can't just create
a cast that only kicks in on empty strings whilst leaving casts of other
strings as is...

Thanks

James