Friend,
Have you used the command default-information originate under OSPF process
Here's one way of achieving this
use distribute list and send only the default route out of the interface connecting to the inside networks
Say your router is sending the default route to internal router R1 and is connected to fa 0/0
On R1
router ospf 100
distribut-list 1 in fa0/0
access-list 1 permit host 0.0.0.0
HTH, rate if it does
Narayan