I haven't used this often, but I remember in 11g you can add properties in the composite.xml, get the values using an xpath expression, and change the values on server using the MBean Browser at EM.
But I'm not sure if the property name should start with "bpel.preference" or just "preference".
You need to check this...
Some references (you will find many more searching on Google):
** check this if is still necessary do that in the current server version, I'm not sure.
Luis F Heckler