I'm not really involved these days - real life has got in the way - but I'd venture that there are no immediate (or even mid term) plans for it.
Possibly after 290 & 975 are finished, although I've heard it rumoured that a certain 70 footer might happen then.
It'd make sense to do (6)820 in tandem with long term back-of-the-shed resident (6)824, with 820 being restored as NG & 824 getting a BG chassis. That would plug two gaps - Centre Luggage and the 1892 Gauge Conversion.
There will be plenty of clues for those that can be bothered to look - partition locations & thickness, upholstery depth, luggage rack support positions, picture rails, ceiling panelling, steam heat radiators & piping to name but some.
Didcot's (6)824 has some rather neat let-ins in the roof where it was converted from oil to gas lighting, and the remains of the luggage compartment has areas of graining/scumbling. Underneath, the longitudinals (stringers/bottomsides) feature two sets of large bolt holes, the second of which dates from the gauge conversion.
Pete S.