> Is there a way to edit the in-kernel copy [of the disklabel] only?

I believe so, from an API/ABI perspective; check out DIOCSDINFO and
maybe DIOCKLABEL.

However, I don't know of any command-line tools that use it (without
also setting the on-disk partition) except for mine, and mine doesn't
grok fdisk-style MBR partitioning and hence is useless for what you're
trying to do.  (If you want it as an example of using DIOCSDINFO, of
course, I'll be happy to send it to you.)

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