check out http://www.pdflib.com/pdflib-cookbook/pdf-import/stamp-pages/ which place a stamp across the page to all imported PDF pages.
For you case, you just have to adjust the
p.fit_textline("PRELIMINARY", 50, 50, "font=" + font +
" fontsize=1 textrendering=1 boxsize={500 700} stamp=ul2lr" +
" strokecolor=red strokewidth=2");
with the data and the position you need. For details about the fit_textline() options, check out the PDFlib API reference.