The mentor legion was one of my favorite chapters in 40k. Has anyone created them for DW? Given their tendency to operate as mentors to other imperial forces it seems pretty natural for them to serve in the death watch.

As a matter of fact, yes. Alex (ak-73) , a prolific contributor and one of the Ancient Ones of the DW forums, created comprehensive rules on his blog (which is generally worth checking out for DW players and GMs alike):

http://40kroleplay.weebly.com/40k-roleplay-blog/chapter-rules-mentors

thank you, brother. Thank you very much. This was wonderfully helpful to me.