Greetings to all, thank you for your interest in this post.

We would like to extend K2 ACL to add a view permission to some categories of our site. This would secure the items contained in these categories using a login. User permissions would be preferred as opposed to group permissions.

Has this been done before?

Please help, thank you.
Regards

You can do this with Joomla! 's ACL.
Each category level has an access level.

You can set it to the Joomla! user group you want.

Are you talking about Joomla 3.0 ACL?

Are you talking about viewing permission or editing permissions?
Because from what I know, joomla doesn't have any viewing permission. The articles or category are either published or unpublished, and only editing permissions are possible.

However backend components access can have viewing permissions.

Am I wrong? Please kindly correct me if I am.

Regards

In K2, each category and item has an option called "Access level".
The same applies to the menu items, but this a different story.

In that field you can set the lowest Joomla! (not K2) user group that can access that piece of content.

These options are not located in K2's permissions. They are located under each category's settings.

Is there any support that has been done so far for user permissions (as oppose to group permission) in the same fashion as we are treating right now?

I don't think this is necessary since you can make as user groups as you want and assign these users to these user groups.