fetch preseed file through a http proxy

is it possible to fetch the preseed file through a http proxy server?
if so, what kernel boot options are required?
i know i have to use the netboot initrd image and not the standard initrd.

our bootstrapping:
- boot with static IP configuration from a minimal ISO (kernel+initrd), this works (the netcfg/get_* stuff as kernel option)
- fetch preseed file through http proxy, this is a hard requirement (url=blabla)
- fetch distro also through http proxy

anyone? :)