Well, the good news is that according to the SZGH 1000MDb manuals, G28 is supported. I would have guessed that it is because, supposedly, the controller supports toolchangers (although my project isn't using one). I think that G28 can often translate loosely as "move to tool change position." Still good to know that they make specific reference to it.

Bad news is that I can't find how to actually set the G28 position. There are hundreds of parameters that are settable and I'm guessing that it's in there somewhere but I haven't found it yet. A search for "reference position" in the two manuals throws up lots of references to how and where it is used, but nothing on how it is set in the first place! But we'll get there eventually...