i got the same reply from american steel when i checked with them last week about storage. our camp got an art car a couple months ago, and we're in the same situation (at nimby, but exploring other options just to see what's out there).
even if no one knows of specific places that would be good for art car storage, general tips on searching for warehouse space that'd fit an oversized art car would be welcome. i've never had to deal with anything storage related outside of the usual handful of boxes that take up too much space in the apartment. we're in SF, so the peninsula would be just as convenient as the east bay.