ISNULL(field, value) is not supported at the moment... so substituting the value with 0 or something is not doable.
Another way to do this (in SQL that is) is using UNION. You can simulate a UNION by doing two entity collection fetches using the same filters, except the first is with an FieldCompareNull predicate where the field shouldn't be NULL, and the second, fetching into the same collection, is with the predicate filtering on the field should be NULL.
if you're using SelfServicing, be sure to set collection.SurpressClearInGetMulti to true before calling GetMulti() the second time.