I always fit them in the mobility installations I carry out (mandatory) and in domestic properties where there are kids or vulnerable adults, I have always bee able to persuade parents/carers to have them fitted, when I mentioned them to the owner of the salon she was able to pull her H&S book out and showed me that her staff are trained to spray on to the back of their hands rather than directly on to clients scalps/hair so that any over temp would be immediately spotted, and her manual was approved during last local authority visit (something she was very proud of) I did respond with the old duty of care to employees bit but didn't get far with that.
BTW no stored hot water on premises, hot water provided by 2 separate non thermostatic electric showers (conventional Tritons 8.5 kw) one for each basin, but they have struggled recently with the greatly reduced temp of of the incoming water to create a good rinse volume at reqd temp. I have offered several solutions/improvements, but cost is a factor and the owner is watching the pennies. Many thanks for chat, I shall still continue to strongly reccommend TMVs but will put disclaimer on invoice if not fitted.