There isn't anything out of the box that could accomplish this. A customer could probably develop something custom along the lines of:

Agent determines that they need to call a customer back

Agent presses Task Button assigned to Launch External Application action (launches a custom app)

Custom Application prompts agent to enter desired date/time/number for call back

Custom Application stores agent input

When time for call back arrives, Custom Application sends IPC message “High Priority” message to the Agent Desktop (format described in Admin Guide)

Agent Desktop displays High Priority message.

I'm not sure the amount of work it would take to create, and maintain, the custom application. But if needed, this is something the customer could investigate.