Hey there! It's been a couple of years, but base-installer and debootstrap are still in this odd state where base-installer wants the script to be just the distribution and debootstrap wants it to be the full path. So, whatever you put in the preseed you get an error (alas, the base-installer one isn't fatal, so at least that works).
Tianon's patch is really simple, and not only does it fix the problem, but also makes preseeding more flexible. Can it be applied please? -- Thanks! Marga