There are item-level permissions that can be set, so you can override list-level permissions on an item-by-item basis. Adding this functionality into your existing workflow probably makes the most sense, but of course there is nothing out-of-the-box that SharePoint provides you.
Luckily, you can extend SharePoint's workflow by creating your own custom actions. The process for doing this in SP2010 is fundamentally the same as 2007; check out this MSDN tutorial for an overview of the process.
There is also a convenient package of custom activities provided in an open-source product called SPDActivities at CodePlex. Specifically of interest to you is the Grant Permission on Item activity. Even if you opt not to use the whole package, you can examine the source code and see about implementing your own version of it (I did something similar for a past project).
Once you have a workflow action for setting an item's permission level, simply add a step to your existing workflow to set Read
permission for the affected audience or group.