Expand pixel selection to grid lines

[TheBladeRoden]

Is there an automated way to expand a pixel slection to fit the grid lines? Picture example included to explain myself better

[c.pfaffenbichler]

If the grid is square one could switch to Quick Mask, Apply the Filter Mosaic and the apply threshold. 

That could be automated with an Action or a Script. 

[c.pfaffenbichler]