There is actually a national standard for the attachment points and expected sizes of toilet seats. In the US, at least, there are only two sizes of toilet seat: round and elongated. Every toilet has the holes for the seat attachment points in the same place and layout as every other toilet. [Edit: As HJ points out below, this isn't 100-percent correct, because certain designer-series toilets have custom seats. But it is generally true, and, most importantly, the location of the seat mounting holes does not differ by toilet brand, with the exception of toilets that come with custom seats.) So a Bemis/Mayfair elongated seat, for example, will fit on any brand of standard elongated toilet. However, the area behind the bolts, i.e. between them and the tank, is not standard, and the tank may curve or the area between tank and bowl may curve up (i.e. in a one-piece). This may preclude the installation of a bidet seat.
So it shouldn't matter what brand of toilet you are using, just how it is laid out. Once you know WHICH Toto toilet your developer is installing, you will be able to know for sure whether the C3 will fit; just call Kohler and ask. (Edit: I see that your developer is installing an Aquia II. So call Kohler and ask.)
That said, why not go with the ORIGINAL bidet seat: the Toto Washlet? It is expressly advertised as fitting on any standard toilet. Folks in Japan have been using the washlet seat for decades. See the selection at totowashlet.com or totousa.com Toto also makes an expensive toilet with a built-in Washlet known as the Neorest. Very popular in Japan, and appearing more and more in high-end bathrooms here in the US.
Toto has a chart about whether a particular Washlet will fit on a particular Toto toilet. http://www.totousa.com/Portals/0/Bro...omp-1009_2.pdf Bottom line is that everything but the S400 will fit the Aquia II.
As an aside, your developer is doing the wisest possible thing by giving you a Toto toilet rather than a Kohler, in my opinion. I have had horrible experiences with some beautiful-looking Kohler toilets not offering a good effective flush. Any Toto model is going to give you a great flush, and use simple, standard, available-everywhere replacement parts when the flapper or fill valve components wear out in 5-10 years. (That said, the Aquia has a different, dual-flush, valve, although it uses the standard Korky fill valve.) No gimmicks, just great engineering -- from a model you can get for about $200 to their snazziest gravity toilets which cost in the neighborhood of $800.
You also might have some fun looking at different models of Toto toilets here on Terry's web site or on totousa.com