not AFAIK, FileMaker needs to go via page setup / preview to gather that information - including object-sliding
would be nice if there would be another method
Markus is correct. If your report layout has sliding objects on it, there's really not a good way to evaluate the number of pages without Preview mode. If they insist on having this functionality in Go, about the best you can do is make the layout without sliding objects and then predict the number of pages based on how much space each record takes up (which you can do through the desktop client).
You can view the PDF of a printout in Go and then count the number of pages in the PDF. The user could then enter the total number of pages into a global field that will print on the printout. Not a perfect solution but it will work.