On the other hand, it might be good to bring back full chat functions for custom matches and random 4 vs 4 practice matches, outside of ranked matches, as it would allow for players to discuss and develop deeper strategies.
In the light of this, it's kinda moot to allow mentors to bypass the chat restriction. You can have everyone bypass it outside of ranked, why settle for less? Also note that mentioning "Random 4 vs 4 practice matches" separate from custom matches implies such a mode is on the way.

The chances that they'll allow chat ever again in Ranked matches is slim to none. Too many fall for the pretense of "serious competition" it purports to offer, take it way too serious and tilt over it, which is the reason the restriction exists in the first place. The best you can hope is that the premade Ranked option they're working on will allow the team to chat, but solo queue will most likely never have it again - not mentors nor anyone else.