I'll answer my own question - in file pd.tk, at the end of the "main window set up" section, after lines
wm title . "Pd" . configure -menu .mbar -width 200 -height 150
add the following line to control main window position at startup:
wm geometry . +x+y
where (x,y) is the offset in pixels from the top left corner of the screen. So "wm geometry . +0+0" puts the pd main window in the top left corner of the leftmost screen.
The relevant Tk manual page is: http://tmml.sourceforge.net/doc/tk/wm.html