Tim,
I would be happy to discuss this with you at length at anytime, and even make the cross you ask for and provide the results for you, I should note as I stated earlier Ron and I have followed very different breeding practices, I have not incorporated crosses and back crosses into the birds I have they have been line bred in two distinct lines one un crossed which developed the blue downed with white breast chicks and tends toward light leg color. The secind was the group I crossed with white and seperated the two there.
I have aquired a light legged female also from Ron's matings to confirm what might be achieved by a line cross at this point and have no young of age to know at this time if that even worth persueing.
I have Fawn Silver Duckwings available to me for the purpose you describe as well as the "sport" from Fawn x Fawn matings I do not beleive this will yeild the result you expect as Dun much like Blue require black to act on to developthe color and the Opal is not black in any part of the plumage.
I would think it would be worthwhile to determine if any of the birds that were used in the earlier test matings were from an Opal Cross bred back to an Opal and potentialy only 3/4 Opal while showing the phenotype possibly not being of the soundest genetics for the purpose used.