Parking at the Bella Coola Ferry is within Bella Coola Harbour Authourity area. They may allow you to park overnight near the BC Ferry dock but it much preferable to email first with expected day of arrival and they can advise.
Last time I was there in June 2012, I was allowed to park (not camp) overnight there. However friends who were there a month later were not allowed to park.
Contact info:
Email: bellacoolaharbour@gmail.com
Phone: 1-250-799-5633
Note that in the off season until June, the office hours are only half days (mornings usually) so take that into consideration when calling or emailing.
Regardless of where you spend the night, before you leave, take the short drive to the Clayton Falls Rec area. Some interesting and bygone history to see along the road and area.