At first glance it doesn't seem very hard to do something like this
on kernel install:
ln -s raid456.ko /lib/modules/2.6.20-1.2933.fc6/kernel/drivers/md/raid4.ko
ln -s raid456.ko /lib/modules/2.6.20-1.2933.fc6/kernel/drivers/md/raid5.ko
ln -s raid456.ko /lib/modules/2.6.20-1.2933.fc6/kernel/drivers/md/raid6.ko
We could have left mkinitrd alone if the %post script for the kernel that
combined those three modules into one had done this.