About this wishlist bugreport. somewhere in d-i there is "nobootloader" it is used to satisfy the the dependency on a bootloader ( silo, milo, lilo, grub, delo ) It is used there is another bootloader available then debian-installer has to install.
Lets call this nobootloader "havepartitions" havepartitions knows[1] which partitions are available for what. And prepares them to the same level as partman does. HTH Geert Stappers [1] it gets a recipe
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