If you go to Daniel Curran's site and click on the 03 California ralley there is a better picture of one on Billy's car. As far as the antenna in general goes I am reminded of a statment from a EV1 brocheure I have. It went something to the effect of a radio antenna would have reduced range by 1/16 of a mile so it was integrated in to the alluminum roof. If someone was really really good they could remove the roof and shave off the antenna hold and relocate it somewhere else.
Billy inspired me to add the S2000 antenna last fall. It does screw right on in place of the original and I didn't notice any change in radio reception.
That is the correct number that you have listed. On HandA's site under S2000 stuff they offer an Insight antenna for S2000 owners!
Another option from Honda might be the antenna on the Element's and Civic SI's (I think it is one in the same). It looks similar to the S2000 but is smooth all the way to the end. In other words, it doesn't have the little "tip" if that makes sense. I didn't pursue it any further so I don't know if it would just screw on as well.
No. 166, I went and looked in the materials I have. I know I read it somewhere, but can't find it at the moment. I have the paddle shaped one and the big paddle shaped brochure along with a Gen 2 brochure. I can't find it now. I know I read it somewhere though, perhaps on the web site. Or it might have been in the 1997 EV1 Mint condition Owners Manual I have . If I find it I will let you know the exact wording.
As far as the other antenna's I would think the other antennas would fit no problem. The only requisite is that the thread is the same, which I think it should be for most Honda's. And I know exactly what your talking about with the similar Element antenna.
I am very tempted to order it to try on our Insight (it's only $9)... The VW guy who installed it on his New Beetle says it did not reduce his reception, and it looks to be no bigger than 2" or so in length. It's all metal with no knob at the top, brushed metal finish, and looks to be veeeery cooool...