I realise that, hence my wording - but, even if it is irrational, it just doesn't feel right to me (and I suspect others) to have a ('MF') JB embedded in plaster. I have to say (and more people may differ from me in this respect) that I'd also feel very uncomfortable with (equally permitted) crimped joints buried in plaster.The implication of the regulations is that there is nothing whatsoever wrong with using a junction box which can never be accessed.
Kind Regards, John