You can just set the GAL filter to be the string "zimbraAccounts". As I mentioned, the server will check all the zimbraGalLdapFilterDef values, and see that one of them starts with "zimbraAccounts:", and use the value after the ":" as the filter value.
The GAL LDAP filter is a standard LDAP query that will be made when user enters a search string. The value "%s" in the query will be replaced with whatever the user typed in.
As for the search base, unfortunately I think you currently can't set it to "", which is what you need to do to search across .com and .ca. You can't set it to that because LDAP doesn't allow attrs with a value of "". When I fixed the bug in our main branch, I special-cased a few values, and "ROOT" will be equiv to searching at the top of the tree.