How can I get a total page count of a PDF before placing every page in the PDF?

Before the [long] spiel, I'm using javascript in InDesign CS3.

I'd like to create a script that places a multiPage PDF in any number of different impositions. (saddle stitch, 4up signatures, 16up signatures etc.)

I've easily created a script that iterates through a PDF and places it into a new document as (4,1|2,3), (8,5|6,7) etc, which works for printing in duplex, folding each page in half, and gluing the resulting spines to make a simple thick book (for PDFs with more than, say, 64 pages).

However, the next step is to re-write the script to create a saddle stitch document (16,1|2,15), (14,3|4,13) ... (10,7|8,9). For this I need to know how many pages there are in the PDF before I start placing the PDF pages, and then making the new document [int((PDFpages+3)/4)] pages long.

Is there a simple way to get the count of PDFpages without going through a loop and placing the next page from the PDF until the placed page number equals the first page number?

This way seems wasteful:

var totPDFPages = 1;
app.pdfPlacePreferences.pageNumber = totPDFPages;
myPDFPage = sheetFront.place(File(srcPDF), [0,0])[0];
var pdfFirstPage = myPDFPage.pdfAttributes.pageNumber;
while (doneCounting == false) {
totPDFPages += 1;app.pdfPlacePreferences.pageNumber = totPDFPages;
myPDFPage = sheetFront.place(File(srcPDF), [0,0])[0];
if (myPDFPage.pdfAttributes.pageNumber == pdfFirstPage) {
totPDFPages -=1;
doneCounting = true;
alert("PDF has " + totPDFPages + " pages!");exit();
};
myPDFPage.remove();
};

NB. Javascript above *hasn't* been run, but should look similar once debugged.

The only thing I've though of to relieve the sheer duplication of placing the PDF twice (once for the count, and once for the imposition), is to create an array of impoPages[counter]=myPDFPage, and then shuffle the pages referenced by the array to the correct sheet and position.

It'd be much easier to be able to assign pageCount = File(srcPDF).pageCount !!!

Thanks for any help/tips or even a simple "What are you smoking, man!?"

Cheers,
Jezz