I noticed recently that New Washington St. (off Washington, just east of the tracks, past the Cafe Belo) looked like it had signage allowing overnight parking of commercial vehicles, which I was surprised by as I didn't think Somerville ever allowed that on any public street at all. I'd check out the details before assuming anything past that, but it's a possibility. (It's also a really quiet and perhaps mildly sketchy area, fwiw.)