If the host doesn't do so in a timely manner I would consider sending a polite email out to the whole group. Something along the lines of "I understand that it is sometimes difficult to find appropriate parking and that many times parking can be inconvenient to your ultimate destination; however, as a cyclist and your friend I would like to point out that parking in a bike lane endangers cyclists and is illegal. I would encourage you to seek out other, more appropriate places to park while attending the potluck. Thank you."

I don't think I'd go so far as to call the cops, especially since these are friends and friends from church at that and the law is less than clear at the moment. I may be tempted, and I'd be upset/disappointed in the host if she did nothing, but in the end it's not worth the fallout and probably not the right thing to do either.