The first thing I tried was to change the plugin so it extends the Point class instead of the Dummy. And then accessed the delegate's drawOnTop property. This didn't cause an error but didn't have any effect either.
In the mxs documentaion > point helper, it says
'When this is on (drawOnTop), the point helper object draws lines with the Z buffer turned off '
I couldn't find anything in the doc. that gives access to the z-buffer when drawing a particular object (this is probably badly phrased).
I finally looked into the viewport drawing methods and again couldn't find anything on a per-object basis.
Does this mean that the only way is to draw the object manually through a callback function, which means registering a callback for each instance of the helper?
Or did I overlooked something simpler?