It is peculiar that the Message command does not actually display a message (it sets a translation/mapping of an existing message, for future use), and that there does not seem to be any other way to dynamically output from the script to the display screen (except via the statistics window which is pretty awkward and limited).
However, it is possible to use the existing Message command in a tricky way to display a custom Pause message:
Message("Disk %1s Paused, press space to continue....",
"Disk %1s Paused, about to place: MFT at 10%....")
PlaceNtfsSystemFiles(Ascending,MftSize * 0.1)
This trick can be used over and over in one script to keep displaying changing messages.
Sadly, there does not seem to be any way to insert any custom variable data into messages this way, beyond whatever parameters are already included.