@Tomohare: maybe, if it suits your use case better, you can try another approach to make the query only on user column even when the `use_domain` is set, via new function `sd_lookup_group()` or a new optional parameter to `sd_lookup()`. So there will be one function to do the query on both user/domain and the other one only on user column.
The function take the db table name as a parameter, so you can use two db tables to separate the group and user-specific records (`sd_lookup_group(groupdial)` and `sd_lookup(speedial)`) . You can also set the name of the column to something that suits better both cases (e.g., `owner`).
From my point of view it felt that the module offers what you need, but it is fine to add new function/functionality for more flexibility, but I didn't want to change the database structure unless really needed, because that has impact on upgrades (even on major release upgrade, but better to avoid when possible).