I wrote a new tool called SMAK that loads obj and sia files and generates quick and easy AO maps. Here's the website: http://matreyastudios.com/smak I designed it to be very fast (it takes less than a minute) and simple to use. It uses hardware-accelerated shadow mapping to get the job done, which is why it's so fast. There's a video on the website demonstrating it. Here's an example:
<img src="http://matreyastudios.com/smak/front-side-ship.png">