Yes, St. Charles did/does have a small museum area in their City Hall with a nice diorama display of the Battle as it centerpiece. It had been about 10 years since I've been there myself. But, I don't remember a dress being displayed. They may have moved their museum to a different location and enlarged it to include the dress, if they have it.

St. Charles was actually occupied by a federal naval expedition a second time right after the battle of Arkansas Post to destroy gun emplacements there that were being built again. But there was no military actions there at that time, as the Confederates had abandoned the place prior to the federals arrival.

There were also several expeditions from Helena by federal Cavalry and Infantry to the White River during the summer of 1862 and in 1863. These expedition were primarily directed toward Clarandon, AR. with an objective of going on to Little Rock, but never when that far. In any case Federal cavalry during these expeditions did occupy St. Charles for periods of several hours to a couple of days. This incident could have occuried during any of these time also.