Details could be found here:
http://sourceforge.net/mailarchive/message.php?msg_id=24546633
The new information tag (isc_info_sql_explain_plan) can be used to retrieve the detailed textual output. Please note that it's still limited to 64KB, due to the API restrictions, so the output could be truncated. An unlimited version of the text would be available via the MON$STATEMENTS table.
Is this still pending to be implemented?