It's still the same problem. Typically when other domes are unavailable I see appraisers using similarly sized A-Frames or log homes for comparison, the idea being to compare the subject to other unique, custom properties as these should have similar market appeal, even if they don't look the same. One thing to consider is the market can usually be expanded as the buyer for such a unique property probably wouldn't confine their market search to just a few miles.
Interesting room sketch.
Terrifying sales grid. Hope this is helpful.