An issue with that is how to deal with things that want random numbers before the disk is writable. If you read the seed while the disk is read-only, and then something uses random numbers based on that seed, and then something causes the boot to abort before the on-disk seed is updated, then the next boot attempt will reuse the same on-disk seed.
Depending on what random numbers are used for between seeding and writing the updated seed, that could range from harmless to disastrous.
With some care in how things are ordered during system startup, it could probably be made safe.
As long as you order it so the network isn't up before the disk is writable, then no persistent effects could happen from reusing numbers, right? If you can't send out a network packet and can't write to the disk, what other recordable events are there? An attacker writing down numbers observed on the console?
Depending on what random numbers are used for between seeding and writing the updated seed, that could range from harmless to disastrous.
With some care in how things are ordered during system startup, it could probably be made safe.