Various methods of filling in PDF forms which are just normal PDFs
oocalc
pdftops
ps2eps
In oocalc:
- Set margins to 0.0 all round
- Disable header and footer
- In oocalc insert sig.tiff and send to background.
- Then insert the page to be signed EPS and send to background
- If necessary resize background to A4 size (21cm wide, keep ratio)
- Align cells and enter text
- 
Export as PDF (just one page) pdfseparate orig.pdf separated.%d.pdf pdfunite separated.[123...].pdf signed-page.pdf signed.pdf 
ToDo
- Fix transparency of sig.tiff