Is it possible to affect the amount of tasks cities do?

[Panzeh]

For example, could I mod it so that invasion forces are more common?

[Rahly]

Its probably possible, although no one here has done it. The scripts that pirates uses are compiled in Gamembryo.

http://www.ndl.com/

although, there tool sets are.... slightly expensive. So unless someone makes decompiler/recompiler, you might be outta luck.

[Sen-Dog]

That would certainly be an interesting adjustment to the game (ie. the % chance / delay between cities releasing types of ships)