It would be very useful if you could add a default data row for a dimension table in a similar way you add custom data but without getting a custom data table and where you can set DW_Id = -1, which is common. The input ui could be in the same dialog as the custom data input or a separate dialog.
The ways to do it now are either:
1. Creating a post-deploy script for each dimension table where you do SET IDENTITY INSERT ON and then an insert statement with all the values.
2. Adding a new custom field to use as a surrogate-key column, persisting dw_id from staging and inserting default values with custom data option.
Numer one is very un-DWA and boring to maintain and number two is somewhat going around things to get the desired resaults and having a custom table for each dimension table just to hold the default value is maybe unnecessary.
Attached image is a quick mockup :)