I'm not sure I completely follow the routing topology.
Does the SRX iBGP peer to all the remote sites have the default route to the ISP in its route table?
If so, you could likely just add an route export term to send this default route down to the iBGP peers. Just insert this term into your existing route export policy.
set policy-options policy-statement YOUR-EXPORT-POLICY term default-export from route-filter 0.0.0.0/0 exact then accept