IO540 its not unknown for pilots subject to FTL schemes to do the odd check ride on days off which would stop being days off if they logged an hour as an instructor. The fact you could spend the whole of your days off teaching gliding (as some do) is quiet acceptable. Your quota is 900 a year and 100 every 28. And there is a heap of rules about manditory days off.
So if you had a prang on on day 2 of your roster and they discovered you were moonlighting for cash on your days off as an instructor but failing to log the hours you would be in deep poo.
And yes Whopity is quite correct it is illegal. My comments on not logging it were in the case of the person being inside 90 days pax currency.