When the VirtualBox bugs (ticket about the guest additions, Debian bug about the host software) with 32-bit userspace + 64-bit kernel are fixed, reconsider shipping an amd64 kernel, and replacing sdmem with memtest.
Until then, ship a amd64 kernel, don't boot it, only use it for kexec'ing to memtest.
No. Let's wait for
feature/hugetlb_mem_wipeto be merged. If we're happy with it, we can avoid moving to memtest.