I would definitely love them in Runner form! The Ashton's Runner duck book has a photo of a gorgeous Overberg, but it doesn't say if it is a regular Overberg or an Overberg Runner. It is being held in the picture, so it is hard to tell. Anyway, it is a beautiful color and yes, you are right on. It is harlequin phase dusky plus homozygous blue dilution, so basically Apricot Silver/Snowy. The birds that Dave created he did so from breeding Silver Headed Australian Spotteds to Miniature Appleyards/Silver Bantams.
I have a lot of extra hens in the Overbergs and extra drakes this year in my Silver Bantams, so I am thinking about breeding some together to create some Blue Silvers with the idea then of the following year breeding the Blue Silvers together to get Silvers and Apricot Silvers to outcross back to the original birds.