On mentats i have simple and good (seems to me) idea:
As there few types of mentats, mentat unit should have 1 ability (1 button added, works exactly like inspiration spell/building in ffh, without building graphics, mentat should remain in city, otherwise building wears off). , named like Mentat Quarter , or Mentat Overseeing or Mentat Seat.
Then depends of of type (there are few), by "casting" this ability in city it build one of (6?) buildings, depends of mentat speciality.
Each building will called like Sientific Mentat Overseeing or similar, and provide:
mechanics 1: +points +%output +GP of specialization
Example: Scientific Mentat : +4 beakers +20% science +3 Scientist GPP (mentat generating gp seems reasonable).
Economic Mentat: +4 solaris +20% income +3 Merchant GPP
numbers are for example, perhaps need tweaking/ balancing, but hight bonuses seems good for mentats/late tech/requirement of 1 offworld contract resource
mechanics 2 way: Building provide 1 free specialist in certain field and +20% to city output in that field. (thats bit different because have more synergies with SE)
So need just 1 button, 6-7 buildings that not require building graphics/icon. (only xml add), and some minor code to link type of mentat to effect of his "casting" (creating Building type that depends of Specialty promotion)