how old is it?
Is it a 1970s/80s frame? what do the eyelets look like, any chance of posting a pic?
I’ve not seen a nagger with a BB setup like that but as long as the cables have a free run with a wide enough radius then you shouldn’t have any problems. The plastic guide on a standard bb just serves to smooth the transitions, it doesn’t do much lubricating, so a sleeve would probably do the trick. maybe one out of a V-brake noodle? 😕