Preset and lock 'account preferences' ?

[RateU]

On the reg form, maybe you can "combine" a dummy fields (disabled) and a hidden fields (unless you don't want to display the fields on the form) by editing the member_register template. Set it checked or not, depends on your default settings. And maybe you need to override the input value if you're not sure whether a user can bypass it or not in member.php
Example:
   
In the scr above, I've checked the 'Hide your email from other members' setting (by default, it is unchecked), and not allowing user to change the value. And I'm not allowing a user to change the 'Receive emails from Administrators' setting.

You can use the similar method for the UserCP.

   

I'm sorry if I misunderstand what you mean.