This can only get worse for Manly. Building a couple of hundred seats here and there will not really solve the problem. Likewise, putting seats on the hill will actually reduce the capacity of the ground - look at Parra, when they put seats at either end of the ground, the capacity dropped from 24,000 to 20,000 as it eliminates standing room.
And, as OHS get stricter, the problem can only get worse. I had a look at record crowds for suburban grounds on Austadiums.com and it really is telling how OHS and individual seating can cause crowds to drop. The only real example of a rugby league ground I could find was Henson Park:
http://www.austadiums.com/stadiums/stadiums.php?id=144
Record crowd 40,000! Can you imagine 40,000 in Henson today? It'd be flat out with 10,000!
Likewise these three AFL grounds from before they rationalized their grounds:
http://www.austadiums.com/stadiums/stadiums.php?id=93
http://www.austadiums.com/stadiums/stadiums.php?id=140
http://www.austadiums.com/stadiums/stadiums.php?id=56
Those grounds ALL have record crowds in the region of 45,000 but now (and admittedly they've been downsized) have maximum capacities of 10,000 or less. Even in their old configurations they would not be legally able to hold 20,000.
How long until some drunk idiot stacks it on the hill at Brookvale and it's deemed a safety risk? Or worse, sues Manly for millions?