How I propose this could work is that the character this mentor might assume is brought in almost like an IC consultant.
They have an IC contracted time (that adheres to the length of this character ooc)
The character is given either matching, appropriate or max ue.
The character's purpose is outlined by the staff to push a direction and guide/mentor PC's that might be struggling with direction or knowledge.
The character is has a very clear directive that they are a consultant and mentor from both IC and OOC.
I think this could help new characters coming into factions without any sort of idea as to what they are really doing. It eliviates hand holding from the staff of those players/characters and allows over arching goals and desires to push in a much more organic way. Further more it allows those that have the knowledge and desire to mentor the freedom to do so in a way free from their own long term characters.
An example might be.
Mid-bie works their way into a faction that has no current PC's. But this faction previously had PC's that were exceptional in the role yet lack a character that can take up position to mentor and guide the mid-bie. So the faction 'hires a consultant' to come in and show this mid-bie the rope, maybe assist with some work but ultimately is there to pass on knowledge.
This same thing could be used to pass on skill sets to new characters, there are skills that are very OOC knowledge based and the actual UE investment is not all that much trouble but can take months or even years to build that knowledge. If there are no viable characters to funnel that knowledge through in an IC way, you get a situation where you have new players and characters moving into these skill sets with huge holes in their knowledge.
So why not have a short term character come, do a week of workshops for a corporatoration, hospital, garage?
I would also go as far to say this would help the staff re-learn things. Because it is apparent that there are things in the game that are lost dark arts that even the staff know nothing about.