You might try a "mount --bind " of your NAS file system over /var/spool/asterisk/monitor if you are sure of it's robustness (make sure you modify any permissions if necessary) , you should probably also add something like:-
cache_record_files = yes ; Cache recorded sound files to another
; directory during recording.
record_cache_dir = /tmp ; Specify cache directory (used in conjunction
; with cache_record_files).
also to your asterisk options