I noticed this blog entry (thank you, main feed!) with a feature request for Outlook, and I shared that request with the Outlook team. One of the PMs in Outlook suggested an idea of how to achieve something close to this by putting the flags on a toolbar and adding custom names to those toolbar buttons:
1. Right click on a toolbar and choose customize2. On the Toolbars tab, click New and drag that toolbar to just below one of the existing toolbars3. On the Commands tab, click Actions and drag "red flag" "blue flag" etc up to the new toolbar4. Right click on the flags in the new toolbar and choose "Image and Text"5. In the Name field, put in the name for the flag (such as the priority of the followup response, the project it applies to, etc). Put & before the key you want to use for the accelerator - i.e. &X would make it be activated with Alt+X.
P.S. Office's customizable toolbars rock.