It appears (from observation and searching) that SP 2007 will not send a task assignment email when the assignee does not have read access for the assigned task and there is no warning when assigning a task to someone who will be unable to read it.
I have not been able to find any mentions of attempting to address this quirk by either an item event listener or a workflow that would either prevent the assignment or task the originating user to fix the issue by either changing the assignment or having
them added to the appropriate group. I could see one done badly causing a cascade of tasks.
The approaches that I've thought about are:
Attempt to block assignment by hooking ItemAdding and ItemUpdating on the task content type. In ItemUpdating, you could use SPListItem.DoesUserHavePermissions since the SPListItem should be there, but you'd may have check the list since I don't think
SPListItem is available at that time. I could see this causing problems when the assignment is via workflow or something that doesn't like other code interfering with its task creation.
Hooking ItemAdded and ItemUpdated to assign a new task to the assigning user to either change the assignment or have the assignee added to the users with access to the site.
Define a workflow which could be enabled for a list that would check if any new or modified task is assigned to users with
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