What I was able to do is saving a Xcode storyboard in my Xojo project folder (let’s call it LaunchScreen.storyboard). Then, put the following script BEFORE build step in your iOS build settings:
Var command As String
command = "cd ""$PROJECT_PATH"" &&rm -rf ./Base.lproj/*"
Var result As String
result = DoShellCommand(command)
command = "cd ""$PROJECT_PATH"" &&ibtool ./LaunchScreen.storyboard --compilation-directory ./Base.lproj"
result = DoShellCommand(command)
Then, add a CopyFile step to your Build Settings, but AFTER the Build step. Configure it to copy file from ./Base.lproj/ to the home directory of your app (because Xojo will put its storyboard in “(app root)/Base.lproj”, and this will copy the whole directory to the app root directory).
The script is to recompile the LaunchScreen.storyboard every time, and the CopyFile step will substitute the LaunchScreen.storyboardc in (app root)/Base.lproj. Though I put the script before Build, I believe this can work as long as the script is before the CopyFile step.
Using this I was able to create a launch screen supporting both normal and dark mode (which Xojo itself does not support, and I hope it can support this someday). In fact, I also wrote a script before Build step to xattr -cr the project root in order to prevent the annoying dialog box telling me there is something wrong with the finder (which Xojo itself should do, or at least give an option).
Hope this helps.