Это могло быть релевантно?
https://bugzilla.redhat.com/show_bug.cgi? id=484833
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=501465
http://lkml.indiana.edu/hypermail/linux/kernel/0710.2/0295.html
Varnish honors the cache-headers sent by the backend - so setting appropriate expires-timestamps on your backend would be the best way to solve it.
If that is not an option, you would need to either develop a vmod, or implement this in in-line C within the VCL.
Developing a VMod, or writing in-line C in VCL is probably outside the scope of Serverfault, and should be brought up at Stack Overflow.