Automatically draw lyric extenders

[Michael Käppler]

Hi all,
a few days ago I asked this on the -user list and got no response:
If a melisma occurs on a word's last syllable, I want lilypond to 
calculate the length of the melisma and automatically decide, whether an 
extender line is appropriate or not. The user should be able to enter a 
value for melisma length (f.e. four quarter notes), above which an 
extender line is drawn.
Can please someone give me advice how to extend lilypond in a way that 
makes such a thing possible?
I assume a difficult point is to measure the estimated melisma length 
>before< the extender line is drawn.

Cheers,
Michael