Grepping kernel sources for bd_claim indeed shows this is perhaps what I'm looking for.

But how to use it from userspace?

fs/block_dev.c:

int bd_claim(struct block_device *bdev, void *holder)
{
int res;
spin_lock(&bdev_lock);

/* first decide result */
if (bdev->bd_holder == holder)
res = 0; /* already a holder */
else if (bdev->bd_holder != NULL)
res = -EBUSY; /* held by someone else */
else if (bdev->bd_contains == bdev)
res = 0; /* is a whole device which isn't held */

else if (bdev->bd_contains->bd_holder == bd_claim)
res = 0; /* is a partition of a device that is being partitioned */
else if (bdev->bd_contains->bd_holder != NULL)
res = -EBUSY; /* is a partition of a held device */
else
res = 0; /* is a partition of an un-held device */