Looking at the source, it doesn't look like this is possible, but I wanted to make sure, or see if anyone else had this issue.
I want monit to send email, but I can't run a local SMTP server or have it send to a remote one (long story). What I want is for monit to deliver straight to /var/spool/mail/<user>. This isn't a really hard modification to the source (basically rewriting "int sendmail(Mail_T)" in sendmail.c) but I was wondering if there was some more elegant way.