Add a new useExternal setting to the tooltip config which when set to true, would prohibit the tooltip from actually being shown in the chart, but it would still allow the tooltipRefresh event to fire. By allowing the tooltipRefresh event to still fire, an external non-highcharts tooltip can be created using the information passed along in that event. From looking into the code, this can maybe be done in the Tooltip refresh method?

[code]
// update the inner HTML (add the useExternal check here)
if (text === false || tooltip.useExternal) {
this.hide();
} else {

        // show it
        if (tooltip.isHidden) {
            stop(label);
            label.attr('opacity', 1).show();
        }

        // update text
        label.attr({
            text: text
        });

        // set the stroke color of the box
        borderColor = options.borderColor || point.color || currentSeries.color || '#606060';
        label.attr({
            stroke: borderColor
        });

        tooltip.updatePosition({ plotX: x, plotY: y, negative: point.negative, ttBelow: point.ttBelow });

        this.isHidden = false;
    }
    fireEvent(chart, 'tooltipRefresh', {
            text: text,
            x: x + chart.plotLeft,
            y: y + chart.plotTop,
            borderColor: borderColor
        });

[/code]