The BM options are all good and yes, the current closure of Govetts Leap, Rodriguez Pass and Pulpit Rock tracks unfortunately limits the possibilities.
Still absolutely worth doing for the scenery and sense of remoteness, which is less evident with Ku-ring-gai possibilities.
Although nothing wrong with those either, I've done a fair bit of walking around there and still do on occasion, but views are not on the same scale.
Mainly day walks as crollsurf said, but camping is possible in some places on the GNW. In addition to Brooklyn Dam I know I've passed camp sites elsewhere.
Near Crosslands Reserve comes to mind and I thought also somewhere between Berowra Waters and Cowan?
Actually this may help with planning if you decide that way -
http://www.thegreatnorthwalk.com/Another thing to consider with the Blue Mountains is that it's still a fair walk to/from the nearest railway stations, assuming you won't have a car.
Not a showstopper if you don't mind adding the required road/street/FT walking, or prepared to pay for a taxi.
Accessibility to railway stations is generally easier with the Ku-ring-gai/Berowra Valley area, so that's where it does have some advantage.