The Formativ API wraps all the GroupWise Windows Client APIs and does not use the GrouPWise SOAP API. The Formativ API does support the GroupWise Trusted Application API.
This means you could write a workstation based applet which uses the trusted application login feature to cycle through your GroupWise user mailboxes and run a check for mail which meets the criteria you set.
If the date range condition is met, the applet could send a high-priority email to the user advising the details/action and/or it could log the information and perform a post-process to email/SMS/IM an alert to the associated end user. This approach should address your BlackBerry user requirements.
The applet could be set to run upon GroupWise startup each day or triggered by an incoming email. You can also use the external Formativ API to initiate the applet from an external application.
I hope this helps with the ideas.