I may be overzealous here and jumping the gun but after having read as much as possible into National, State and Council owned forest areas.
It seems that State Forests seem to have more flexibility in allowing for MTB trails to be built.
With the Dam being the busiest track in Oz and Ourimbah, Yellomundie and Appin donks away. And forests around the Northern Beaches area being National Parks with other parks protected by the 'Yogi Bear fan club' rangers.
Its a safe bet to say that there is almost nothing else around the Sydney metro region for such a highly populated sporting group.
Lane Cove is a joke for single trail and Menai isnt too bad but shared amongst motoheads.
But you look at Ourimbah, Awabi and Yellomundie who are supported by the local MTB clubs, therefore supported by NSW Forests as well.

Would it be prudent to ask the office for Cumberland State Forest (Pennant Hills area) to determine whether a sustainable single track circuit be built around that area?

Having spoken to bikers around the area, they have put their hand up to assist in "Build days" with the rangers to work out a good trail system.

If its determined to be a success, this could well possibly become the busiest track in Sydney alleviating the likes of the Dam.
Its central, there's pockets of hectares to pick from and the economic benefits of it would promote the area significantly.

Rob - Do you think this is feasible or am I blowing wind up my ass again?

I know this technically falls out of the Northern Beaches area but I get tired of driving out so far to get a decent single trail ride if the Dam is closed.