I'm not sure that I understand your request that well. I don't think I know of a method that prevents you from selecting certain components.
Could you possibly use simplified reps? You could create a simplified rep in the assembly that includes the manikin and excludes everything else. This would allow you to work in the assembly but only see the manikin that you need to edit.
when you say "lock out the selection" I assume that you want some users not be able to neither change the position or modify the geometry of the components. please confirm.
On Creo Parametric level, with Advanced Assembly extension, components can be set as external copies but this will not prevent changing position.
The ultimate solution is to use Windchill Access Control system when roles can be attributed with given rights. But there again, if a user can modify an assembly but isn't granted the right the change a component, he will still be able to move the component in the assembly. So not you'll find a solution about preventing moving a component yet being able to modify an assembly.