Unfortunately, there is no built-in functionality at the moment to set a default value for a custom asset field. I believe this feature has been requested by another customer as well and is on the wish list, but we don't have an immediate release date.
You could create a trigger for the relevant table (tblAssetCustom) in SQL Server Management Studio, so that the custom field is updated to the value of your choice when the asset is first inserted. This is not something we can provide instructions or support for however, as any custom database changes can potentially break your installation and are made at your own risk. You can also edit the custom field value of all existing (though not future) assets at once, by clicking the Assets link at the top of the console, ticking the upper checkbox and hitting the Mass Change button on the left.