To all Missouri birders who have seen the Ivory Gull and have communicated about it via Mobird-L or to me personally, thanks! I appreciate the great response.
We do now have permanent evidence of this bird's occurrence in Missouri, with documentation from four different people based on observations on the Missouri side of the river, including some great photographs. If anyone else is planning to document it too, that will be good but not necessary. It is species number 432 for Missouri (or 421, not counting provisional species).
By the way, the island called Quinssippi on which the marina (its recent hangout) is located, opposite the Pier restaurant and the Dock, is entirely in Illinois, not Missouri. On the other hand, the western half of the river from the bridges south to the dam IS clearly Missouri. All documentations I've received refer to that area.
I did get there myself on Sunday, with my son Matt, and watched the bird a while, although it was not as easy to locate as it apparently had been on Saturday and flew mostly along the Missouri side. We had to leave before it reappeared near the Pier.
Again, thanks, and good luck to anyone who goes up there from this point on. It's there today.