> Httrack -- or at least the GUI -- is obviously unsuatable 
> for this purpose, but I thought perhaps the library could 
> be suitable... 

Not the library - but maybe a script using the httrack 
commandline:

The idea would, for each site, to use the "Add 
URL"/"Capture URL" feature of httrack, then create a script 
that would modify the "postfile" login file, and run 
httrack perdidically.

This requires some scripting knowledge, however, and is not 
a totally obvious thing to do ..