User-defined coordinate system

I note that Inkscape has the zero coordinates at the bottom left, but SVG seem to use the top left, which is confusing when using the XML editor; granted using it is not the normal way of editing.
Being able to set the point of origin, as noted, would be handy.
Somewhat separately, some objects like circles are measured from the centre, others like rectangles from the top left, but Inkscape always uses the bottom left. Being able to set the object origin on the Tool Control bar like Illustrator would be a simple but great feature addition.

Now that the viewbox can be specified in the gui, I suggest to add one more option to the 'Documents Properties' - 'Display units' dropdown to show the user defined view box coordinates, e.g., 'viewbox'

The rulers and coordinate information on the bottom right would then show the viewbox coordinates.
This should enable to freely move the origin / invert coordinate axes etc.

My use case is to learn how Open Type fonts are rendered. To do this, I need a coordinate system whose origin starts in the lower left corner. Other use cases include PostScript and PDF, which use the same coordinate system.

In the document options, when I saw the "Scale y" field, I thought everything worked. Only to discover that the "Scale y" field is disabled (always), referring me to the Viewport options below. When I click on the "Scale y" field, this area might open automatically, as a guiding help.

In the Viewbox, I can only set the origin and the width and height, but not the scale. (Didn't the tooltip at "Scale y" promise that I could set it here?)

I tried to set the Height to -29.7, but that was "autocorrected" to 0.01. This is a rather unexpected behavior. I expect that the invalid value is marked as invalid (red background, tooltip, separate error message, anything). Inkscape should not autocorrect numbers, at least not to unreasonable values.