Hi Pikey,
The answer is probably, "that isn't really possible right now," but it's a good idea!
This may seem close (and maybe close enough):
<ma-serial-chart style="height: 300px; width: 100%" series-1-values="hwinletPressureValues" series-1-point="hwInletPressure" series-1-type="line" series-1-color="#ffaa00" legend="true" export="true" options="{ valueAxes:[{logarithmic: true, dashLength: 6}], guides: [{dashLength:5, inside: true, label: 'average', lineAlpha: 1, value: statsObj.average.value}]}" ng-if="statsObj.average.value">
</ma-serial-chart>
But the problem comes in where the serial chart will be aware of updates in the set of point values, but it will not adjust the options to match. So, this will draw the line for the average on the first page load. Perhaps one could hack up a solution to redraw the whole component, but that's not ideal. There may be a way to adjust this value more directly in the amChart but I am unsure of an easy way to do that.