This, in fact is *EXACTLY* what we're talking about; it does require autoassemble. Why do we care about the partition types at all? The reason is that since the md superblock is at the end, it doesn't get automatically wiped if the partition is used as a raw filesystem, and so it's important that there is a qualifier for it.
I don't like using partition type as a qualifier, there is people who do
not wish to partition their drives, there are systems not supporting
msdos like partitions, heck even m$ is migrating away from those.
In any case if that has to be done it should be done into mdadm, not
in a different scrip that is going to call mdadm (behaviour should be
consistent between mdadm invoked by initramfs and mdadm invoked on a
running system).
If the user wants to reutilize a device that was previously a member of
an md array he/she should use mdadm --zero-superblock to remove the
superblock.
I see no point in having a system that tries to compensate for users not
following correct procedures. sorry.