KiCad placement using coordinate system
There's not, as far as I know, a command line way to do this. You can do it be editing the .kicad_pcb file directly. Steps are:

In Pcbnew, draw out an outline anywhere. You'll want to use the Add graphic line tool (shortcut Ctrl-Shift-L) and use the Edge.Cuts layer. Save this file, and close Pcbnew.
Open the saved file in your text editor, and search for Edge. There will be entries for the outline you've drawn, starting (gr_line [..]. You can then edit this to give your desired coordinates. Save the file, and re-open in Pcbnew, and you're good to go.

For your board, it would be:
(gr_line (start 0 0) (end 0 30) (layer Edge.Cuts) (width 0.05))
(gr_line (start 0 30) (end 55 30) (layer Edge.Cuts) (width 0.05))
(gr_line (start 55 30) (end 55 0) (layer Edge.Cuts) (width 0.05))
(gr_line (start 55 0) (end 0 0) (layer Edge.Cuts) (width 0.05))

Alternatively, go to (0,0), and just set your grid to 1 mm then you can draw it directly.