Hi,
This is not available to plugins. You can show alerts on a view level (within the plugin iframe).
Globally, the best practice is to create vCenter tasks for operations which are long-running or might fail. In this case the Task panels would provide notification to the user for the success/failure of the operation.
As this is not the first time I hear this question: could you please share what kind of use case you would use the global alerts for?
Thanks,
Vladi