Hi,
When i run world update, I usually don't really check all the written stuff.
If I do this, I'm sure a lot more Gentoo users do the same. 
So do expect people rebooting the machine without checking what your have wrote.
This can be a major headache if you have few systems that are doing auto updates.
I would solve this issue by stopping the emerge and getting the attention of the user.
If I don't get the attention of the user, no openrc will be installed.
It should be something like emerge -C ... 1 .2 3 4 5...

To conclude, you can't issue such a change without proper confirmation from the user.

Regards,
Kfir