On NG there is always the slip indicator. During SE ops, it is not the primary source for rudder trimming, and with a level control column it can be a sniff out of centre. If you absolutely had to trim in a turn I'd be aware of what this indicated for some help.
There is a huge difference in not 'red marking' some one for trimming in a turn if they understand what they are doing, can achieve it successfully without compromising a/c control, and they are aware of the consequences, and actively teaching it. In 'teaching it' that could be to first time jet pilots who might not qualify in all points of my previous comment. Dangerous to take one small snippet of FCTM to over-ride another.
If a pilot can not sustain rudder control for a complete turn I'd rather advocate some gym time than trim time.
should the PF become incapacitated during that early engine failure turn due to the stress, and say have a heart attack, the PM will have virtually no chance of recovering the situation, however if even some trim has been applied, then their chances have improved dramatically.
If that is the best reason to teach a debatable technique........well.....?