Hello James,
I would use a first deny block with a prefix-list that matches only the default
then a second permit block with no match statement and a set local-preference
ip prefix-list only-default permit 5 0.0.0.0/0
route-map all_but_no_def deny 10
match ip address prefix only-default
route-map all_but_no_def permit 20
set local-preference 200
router bgp yourAS
neigh xxx route-map all_but_no_def in
Hope to help
Giuseppe