Ellis Wainwright was born around 1850 and was the president of the Wainwright Brewing Company in St. Louis. He was a rather prominent member of St. Louis society and was involved with the art museum and the public library. Wainwright was also a member of the St. Louis Club and the Veiled Prophet association. In his obituary, it was mentioned that he was one of the wealthiest men in St. Louis.
There can be no doubt that he moved in the same social circle as Henry Lucas and appears to have been friends with several members of the Union Club. Interestingly, he was caught up in the "boodle" scandal along with Union Club member Charles Hunt Turner. While there doesn't appear to be much of a connection between Wainwright and baseball, he was a bit of a sportsman and was particularly involved in horse racing in St. Louis.
I did find two instances in the Globe where Ellis Wainwright was referred to as Elias but in each instance it was clear that the person in question was Ellis Wainwright. Another source that came up when searching "Elias Wainwright" was actually a reference to Ellis Wainwright and was simply an error in the soundex due to the similarities in the name.
Based on the information at hand, I have no doubt that Ellis Wainwright was one of the early backers of Henry Lucas and the Maroons. Any references to Elias Wainwright is merely a typographical error or an understandable misreading of the source material.
Edit: While I'm at it, David also mentioned that there may be references to Lucas' UA plan in the press outside of St. Louis in the summer of 1883. So far the earliest references to the plan that I see in the press comes in October 1883. The New York Times has a story on October 25 that they apparently picked up from the Globe. Earlier St. Louis references to Lucas and baseball have to do with the amateur Lucas Nine rather than to the Union club. However, if Lucas was announcing his plans in October or if the story was breaking then, certainly it's safe to assume that Lucas' plan was in development prior to that. If that's the case then it's possible that there may be earlier references to it and I'll continue searching for them.