I have stickers for organisations that I'm a member of on my car - they get me free parking at some venues. Other people have bumper stickers - either jokes, or showing support for environmental or political causes. I doubt something like that would cause any problems, and might spark some interesting conversations.

But I'd steer clear of a giant-sized paint job that might imply sponsorship or endorsement by the wider BOINC community. Might not look good if you found yourself in an embarrassing position while driving it!