Marking a found pattern is already possible. You can draw a line with the plot function, like this:

static bool DoLine = false;
if(patternstart) DoLine = true;
if(patternend) DoLine = false;
if(DoLine) plot("Pattern",priceLow()-30*PIP,0,RED);

This draws a red line under all found patterns at 30 pips distance from the price.