Is it possible to modify an Outlook MailItem object to display a Mailtip with custom text? It doesn't look to be possible using the OOM, how about at the MAPI layer, i.e. using Redemption?
If not possible, is there some way to achieve a similar effect? (e.g. Ribbon XML, etc.)
The best you can do is add a category - it will be displayed in the same area. The tip itself comes from Exchange server as a result of an EWS call. There is no way to display custom data there.