I don't think you have a very good grasp on what a co-dominant trait is. Stop thinking 'super form', start thinking 'visual hets'. If it has visual hets, it's generally co-dominant.
You're using a few examples that have clear 'super forms'. Start thinking outside the box. Look at rex rats, for example. A rex rat has fur that is sort of curly, a double rex has fur that is curlier. For the human example of a co-dominant trait, curly hair. You could say a "normal" human has straight hair, a "het" human has wavy hair, and a double het, or "super" has curly hair.
eta: this is from Snake Ranch:
http://www.snakeranchnews.com.au/pages/news.cfm?naction=news2
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Hypermelanism, at least in this instance, appears to defy the usual dominant/recessive inheritance rules in that the heterozygotes express an abnormal, possibly intermediate colouration, suggesting a 'co-dominant' inheritance mechanism. Snake Ranch keeper Kevin Smith is holding the founder female in his left hand, and one of the mature heterozygotes she produced in late 2002 in his right."
"[/FONT]Interestingly, upon arrival, the 'het for black' young were not entirely normal in appearance as would be predicted in the usual 'dominant/recessive' genetics model. All were quite dark at birth, and during the months to follow became progressively darker, finishing up as exceptionally dark bluetongues - no doubt qualifying for the description of 'melanistic'. This abnormal colouration for the hets suggests that the genetic control of the condition of 'hypermelanism' (in this case, anyway) may reflect the interaction of 'co-dominant' genes."
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