I have a project where I needed to add mean diamonds to a boxplot, and I have a working prototype where the height and width of the marker symbol are used instead of the radius. The marker height needs to stretch or shrink based on a data value instead of being fixed by a radius value. Below is a shim that I wrote that modifies the Series markerAttribs function. Can you take a look at this function and find a way to integrate it into a future release?
Thanks,
Jeremy

//Taken (and altered) from highcharts.src.js (v5.0.7) line 17497
Highcharts.Series.prototype.markerAttribs = function (point, state) {
var seriesMarkerOptions = this.options.marker,
seriesStateOptions,
pointMarkerOptions = point.marker || {},
pointStateOptions,
radius = Highcharts.pick(pointMarkerOptions.radius, seriesMarkerOptions.radius),
height = Highcharts.pick(pointMarkerOptions.height, seriesMarkerOptions.height),
width = Highcharts.pick(pointMarkerOptions.width, seriesMarkerOptions.width),
attribs;

// Handle hover and select states
if (state) {
    seriesStateOptions = seriesMarkerOptions.states[state];
    pointStateOptions = pointMarkerOptions.states &&
        pointMarkerOptions.states[state];

    radius = Highcharts.pick(
        pointStateOptions && pointStateOptions.radius,
        seriesStateOptions && seriesStateOptions.radius,
        radius + (seriesStateOptions && seriesStateOptions.radiusPlus || 0)
    );
}

if (point.hasImage) {
    radius = 0; // and subsequently width and height is not set
}

attribs = {
    x: Math.floor(point.plotX) - radius, // Math.floor for #1843
    y: point.plotY - radius
};

//use height and width for SVG instead of radius
if (!point.hasImage && height && width) {
    attribs.x = Math.floor(point.plotX) - width/2;
    attribs.y = point.plotY - height/2;
    attribs.width = width;
    attribs.height = height;
}
else if (radius) {
    attribs.width = attribs.height = 2 * radius;
}

return attribs;

};