You mention that the addresses involved are configured as /30 addresses on the AS300 router.
And that what you want is source based routing for these addresses.
You might be able to build the filter addresses then automatically using a prefix list and the apply groups match condition for the interfaces. I have a similar filter build using this basic pattern.
InterfaceAddresses is the name of the list
The <*> matches all of that criteria, in this case all interfaces and all units. You can make this narrower like:
interfaces ge-1/2/<*>
So that only certain fpc or pic are covered where your client interfaces will be configured.
this will then pull all those /30 subnets into the prefix list as the interfaces are configured automatically.
set policy-options prefix-list InterfaceAddresses apply-path "interfaces <*> unit <*> family inet address <*>
Then instead of using from source-address you would use from prefix-list in the FBF example filter. So the filter is automatically updated as the interfaces are added to the system.
