CR can't know where an item is until it has read/written it, so there is no way to determine ahead of time where an item is. It can't determine the results of a formula until it has read the data, how could it determine the number of pages a subreport is going to take?
Depending on your data / subreport and how you are getting the data, I could see a method but it may not be fullproof. If you get the data from stored proc, you could probably determine how many records are going to be in each subreport and if the subreports don't have to grow set on, you can probably determine how many pages (with groupings) the subreport is going to consume, and since each subreport starts on it's own page...
It would add proccessing time to the stored proc, but it might be doable...but there are no guarantees.
here is another way of thinking about it. You are writing a word document, can you write the toc before the document? You can't even tell Word to do it until you are done. CR is no smarter than Word, and it can't go back and add information.
HTH