You could replace the scope with a viewfinder from an older video camera. These are essentially smallish monitors with eyepieces, giving you all the freedom you

You could replace the scope with a viewfinder from an older video camera. These are essentially smallish monitors with eyepieces, giving you all the freedom you want wrt. spectrum displays overlaid over video imagery of the targeted area. Driving the thing might be an exercise in itself but it should be doable from the HDMI port on the Pi using a VGA converter or through a USB display adapter. Personally I'd tone down the gun aspect of the whole thing though... lying on a roof somewhere in the US of A, pointing this thing at a window in an office building nearby is an open invitation for unwanted attention from trigger-happy defenders of the realm.