Visually monitor area of a window in background

Is there a possibility to define a monitor area within/over an arbitrary application window that is in background?

The content of this area either should be displayed somewhere else (always on top). Alternatively a notification should be made on any (pixel) change within the monitored area.

The monitor area should:

• be a free selection
• stick to the window, even/especially if it is in background
• ideally be saved and automatically be reapplied, based on the same window title

The notification should be:

• positioned independently from the source monitor area
• able to be configured always on top
• able to handle multiple monitor area feeds

Similar implementations:

• Compiz window previews in scale plugin
• Cairo dock window previews
• RecordMyDesktop free selection of recording area

Summary: The ideal solution should combine the above mentioned implementations in a new way: free selection of a monitor area (from RecordMyDesktop), live monitor of windows in background (from Compiz scale plugin) and completely different target display of monitored content (from Cairo window previews)

Partially related:

• Record multiple areas on single screen at the same time