Hi Cassie,
First of all, don't panic. I did one of these geodesic homes a few years ago, and they are not that difficult. Obviously, there are most likely no homes like it in the market that have sold. So, I used unusual homes, like a log home, a salt box style, and an earth home, you know, built underground with a walkout front above ground. I found very little resistence in the market for these types of homes, and in fact they may have a little advantage in the market, at least in my area. I found the toughest part was doing a cost approach, and that took a call to Marshall and Swift, for a few answers. In the comments you just have to explain that there are no like comparables, your perception of any marketability problems, and your rationale for using the comparables you found. Just remember, when it is all said and done, how can an underwriter argue with your rationale, when the underwriter has probably had no experience with octagone homes either.
Best of Luck, Chuck