Figured it out..
There is a constructor for LineAndPointFormatter:
setLinePaint(Paint)
So to draw a dashed line, you would then use a snippet like this:
Paint dashPaint = new Paint();
dashPaint.setColor(getResources().getColor(R.color.red));
dashPaint.setStyle(Paint.Style.STROKE);
dashPaint.setStrokeWidth(3);
dashPaint.setPathEffect(new DashPathEffect(new float[] {10,5}, 0));
LineAndPointFormatter redDash = new LineAndPointFormatter(dashPaint.getColor(), null, null);
redDash.setLinePaint(dashPaint);
plot.addSeries(red,redDash);
plot.redraw();