Well, for starters, the Santa Monica Parking Structures have signs that clearly state "No Overnight Parking". Thus, it's definitely park at your own risk in those situations.

As for viable, legal options.. I ended up parking overnight in residential areas. In my case, this was on 2nd street north/west of Idaho St. In these areas, the street signs restricted parking once a week for cleaning (usually a weekday for 2 hrs).

Most other spots, metered and non-metered, required a resident permit for overnight parking (or simply did not allow overnight parking altogether). This is clearly stated in the street signs for parking.

Last but not least, there were a few metered spots that did allow non-resident overnight parking but you had to be up early the next morning to feed the meter. We saw these kinds of spots on 2nd street between Wilshire and California.