No, this is clearly a good idea and something I've certainly done on the L2, after all what's the point of having standby instruments if you can't use them? The only trouble is that they are usually orientated for RHS only and also it's not a mandated item so if you get them to do it and they make a mess you can't fail them. You probably will demoralise them though. On the other hand if they do it well it's a confidence booster. So for me it was a judgement call and only do it if you think the pilot is likely to succeed.
But once again I'd suggest that it should be done in a realistic failure scenario. So for the 225 it's hard to think of something short of a complete aircraft electrical failure that would require reversion to the standby instrument (the standby instrument's data can be put onto the normal screens). So if you are down to the standby instrument only, that means no AP at all. Tricky to then fly an ILS not least because the ILS receiver boxes will be dead! But fortunately an extremely improbable scenario that you would have both a complete electrical failure and a desperate need to fly an ILS.