The tip was about the
TIMESTAMP prefixes for ANSI date and timestamp literals. And when I went looking, I found the documentation in the SQL Reference Manual at Datetime Literals. Things are easier once you know that something exists.
This even works in RMAN 188.8.131.52.0 on Windows XP as the following commands show:
list backup completed after "DATE'2012-12-27'"; list backup completed after "TIMESTAMP'2012-12-27 00:00:00 +11:00'";
Although, it is still better to set NLS_DATE_FORMAT before starting RMAN in order to get the values for the timestamps in a more accurate and readable format.