First, let me confirm, that's already on our wishlist and we would like to implement the Maxscript access to this feature in future versions. Meanwhile, an option might be to use the UIAccessor Core Interface to get the button pressed. The corresponding utility can be opened by using UtilityPanel.OpenUtility RailClone_Tools and depending on the 3ds max version used, you should find the "windows handle" value (it changes) and by using the UIAccessor.PressButton or UIAccessor.PressButtonByName launch it.
Please feel free to get inspired by the following code:
hwnd = windows.getchildhwnd #max "Instantiate"
I hope that helps.