v3 BoundsBox library provides constructors to create a rectangular overlay by LatLngBounds or by LatLng plus km-dimension (also miles or nautic miles). You can also overlay images that scale with zoom.
It is still missing an overlay with pixel dimensions. For tooltips and other labeling. It is coming.
I haven't included the 'tiptool' to BoundsBox library yet. There are a few things still under development.
I am impressed with 'universional' concept of a library that Paul Kulchenko has implemented. He has written some impressive Canvas experiments that work with v2 and v3 with one and only script.
That is the way that Google Maps API libraries should be written.