What I do in this case is create a rule area. Here I can have a different set of clearances within rule area than the rest of the board.
1. In CES create a new scheme. Name scheme for this case "BGA fanout"
2. In "BGA fanout" scheme setup trace width and clearances that will allow you to route out of BGA.
3. Close CES
4. Draw a rectangular shape around BGA
5. edit properties of rectangle select "Rule Area"
6. Select "BGA fanout" scheme.
Now you can route out of BGA while not violating board clearances.
Regards,
Eda