Henry V. Lucas and Ellis Wainwright, St. Louis capitalists, have rented a large plot of ground at Twenty-fifth and Biddle streets, and intend fitting it up for base-ball purposes.-New York Times, October 25, 1883
I'd been looking for the earliest reference to the Lucas-Wainwright partnership and posted that it dated to mid-November 1883. But I forgot about this article from the Times, talking about the establishment of the Union ballpark. While this specific ballpark didn't get built, we can say with certainty that Wainwright's involvement with Lucas and the Maroons date back to at least the end of October 1883.
Also, I had written that Wainwright was involved with two of the more important pieces of St. Louis architecture, the Wainwright Building and the Wainwright Tomb. I don't think it's much of a stretch to add the Union Park to that list. It might not have been designed by Louis Sullivan but it was a significant piece of 19th century St. Louis baseball architecture.