we did a job using monorex G00 about 18 month ago, and simliar to marks job there, the client was adament they wanted them on, anyhow thats what they got, i drive past it now and again, it looks terrible, theres like a black sludge thats absorbed into the mono substrate,and it doesnt help that the elevaton doesnt see sunlight.
the problem is your adding a lot of weight to the original blocks with another 15mm of render to it.
id say wash it down with lanko mousse,to get any impurities of the surface, and acyrlic on a parinter base wth 355 mesh incoporated into it.but if there adament they want a mono finish, thats what they get, all you can do is explain the pitfalls of it.