Okay. I swear I didn't post this just to answer it myself, despite how quickly I got it (always the way when you ask a question!)
Here it is:
ggplot(dat, aes(x=x,y=y)) + geom_point() + labs(y=expression(Blah[1][d]))Thought it best to post the answer rather than remove the question as it may help someone else one day.
'Blahs' aside, what I actually wanted was expression(paste("Hb", A[1][c]," (%)",sep=""))
Why paste0() doesn't work here is beyond me.