Separating the content from delivery: On delivery it has been a historic problem with this thread,that those expressing dissenting views are insulted. It really does not help.
On the content:
Reality is as many have said, the races in which hypothermia is most likely are those in which many or all of the field are wearing body cover. So in the situations where monitoring is of most importance, the over prescriptive rulings on numbers are so many wasted words.
There are alternatives, from tags both electronic and physical, wrist bands and so on, and it is up to the RO to decide what combination of numbering scheme, and checkpoint processes (automated or manual) best suits his race. The rules should specify what needs considering, and what needs achieving, not how it is done, with guidance documents mentioning alternatives for different levels of race risk. The RO needs to put check point instructions in his "runner information" document, and describe checkpoint processes and start/finish processes in his "marshalls information" - so that everyone knows what their job is
And that is how safety documentation is written, with the RO responsibility to assess what best manages safety for his race, and documents it. Simples. Say what you do, then do what you say: which is the best defence against failure of duty.
An indelible marker writing on a leg, is plenty good enough in some races, such as the "fast and feral up and downs" with a few score runners on a summers evening. An unfolded number is clearly useless in others, when hidden below a goretex. Electronic tags work fine in some situations. SHRA have used wristbands before now.