Here is an example for executing rule base syntax on sequence flow condition:
<sequenceFlow id="_9-_10" sourceRef="_9" targetRef="_10" name="sq-name">
<conditionExpression xsi:type="tFormalExpression" language="http://www.jboss.org/drools/rule" >eval(false)</conditionExpression>
</sequenceFlow>
where eval(false) is rule syntax to be executed, please keep in mind that it must return boolean.
HTH