I wrote an article on our company blog about the various ways of achieving this. The Download option would probably best fit what you're looking for.
How to create a SharePoint KPI column
- Designer: Customise the list view using SharePoint Designer
- Download: Download an open source Custom Field Type
- Code: Create a custom field type from scratch
- Enterprise Edition: Use the Out of The Box Enterprise Edition functionality
- Buy: Buy a pre-made Custom Field Type... We just so happen to sell one of these. :)
Links and more information on each of these options can be found in the article.
It sounds like you're going to have a bit of an uphill struggle with the broken permissions aspect: Our product (Highlighter) doesn't cover this functionality.
SPListItem.HasUniqueRoleAssignments will probably be of use to you.