Assuming you want to check the device (extension) status prior to sending the call to it, then the only option is a custom dialplan.
I was actually planning on creating a #wish-list post to request from the developers to add a feature in the Dynamic Destinations modules, to allow Asterisk Variable lookups. So in your case, it would need to be something like:
If ${EXTENSION_STATE(4567@cos-all)} = NOT_INUSE then go extension 4567.
else, then go to the second destination