The fact that it is lethal means that you can prove that it is at least not dominant. Dominance means that the phenotype, as you previously mentioned, is the same whether in heterozygous or homozygous forms. We know that spider is not some weird anomaly that can't have a super due to the fact that black head spider is a super and logically a homozygous form does exist of the spider mutation. Unless there has been some weird backwards bad luck in the past 20 years of who knows how many breedings that the super spider has not been found, logically it would mean that the super spider is a lethal super/homozygous lethal.
Well known people working with the gene include Kevin McCurley who I idolize but does tend to say some things that make no sense at all such as the hidden gene woma granite thing. Kevin believes that the super spider doesn't exist at all and that it isn't lethal either. It just simply does not exist which makes no sense biologically speaking. The best way to prove this once and for all is to do a blackhead spider to blackhead spider pairing and record what comes out. There would only be three possibilities from such a pairing. Super black head, black head spider, and super spider. If you never hatch a spider or something that isn't a super black head or black head spider, then it is 100% confirmed that the super spider is a homozygous lethal. Just because well known breeders say it's not lethal doesn't mean it actually is so given the evidence that has surfaced in the past 20 years.