Something like this should get you close:
dt << Current Data Table();
cols = dt << Get Selected Columns();
counts = NewTable("Counts for "||char(dt<<Get Name),newColumn("Column"),newColumn("Count"));
for(i = 1, i <= NItems(cols), i++,
r = dt << Select Where( ascolumn(eval(cols)) == 0 );
counts << AddRows({:Column = column(dt,cols)<< Get Name, :Count = NItems(r)});
);
I didn't test this.