With parellel plugs it is usually just a slight inconvience at worse if one plug burns out. On that series plug car of ours it's different. I always keep a test light, wrenches and one spare plug in the trunk. A failure of one plug can make the car into a non starter. The chances of someone locally having one on hand when really needed is pretty small..
If really stuck I guess you could undo the heavy bare feeder wires at one open glow plug with the series type. Short them together prefferably on the plug if possible to bypass the one burnt out plug to get you running. You still need a test light or meter to find the open plug though. Or carry a heavy wire with two strong clips to circumvent the open plug. We are dealing with a heavy amp flow here. Use the original size of the series harness for a size example.
Or just carry the right size nut, bolt and washers to temporarily tie the wires together. It's hard to think of quick solutions when confronted with a problem on the spur of the moment sometimes. .
The loop series plugs might be a little more reliable than the pencil type but do fail as well.
I am well aware the pencil type of glow plug set up does seem to do a better job of starting than the older loop plugs. Yet with a good tight engine the series plugs will do the job.
Also I have found over many years experience. If you replace the burnt out plug with a new one. There seems to be about a fifty percent chance it will burn out again at some point before another one of the old ones do with the pencil type. So I have always wondered if it was a quality control problem or some internal cylinder condition exists. Like a different spray pattern on the injector for example. Creating a somewhat different operational enviroment in that one cylinder for the plug.