Printing current date and time on each page
I finally found a usable solution using convert from ImageMagick, the PDF services mentioned by nathang and Automator. 


Start Automator, create a new "Print Plugin"
Drag the "run shell script" action to the right and enter the following script:

export PATH=/opt/local/bin:/usr/local/bin:$PATH
convert -size 400x50 xc:white -font Arial -pointsize 24 -gravity center -draw "fill grey50 text 0,0 '`/bin/date`'" /tmp/stamp.png
cat

Add the "Watermark PDF Documents" action and add the file "/tmp/stamp.png" using the 'add...' button. (You probably have to create this file first, e.g. using touch /tmp/stamp.png in a Terminal). Then adjust the offset, scale and opacity as you like. I'm using x=480 y=820 scale=0.25 opacity=0.7. 
Add the "Open Images in Preview" action. 
Save the Automator script. 


Now you should find a new action in the "PDF" drop-down menu of your printing dialog, which stamps the current date and time on each page and shows the result in Preview, from where you can print the document.