Not that this is much help. but I believe there was a board that could be fitted to the BM7000 for just this purpose - I cannot imagine that one will be available now. One of the reasons B&O removed the RIAA was to allow the Beogram to be sited further away from the Beomaster. By fitting the RIAA in the Beogram, a stronger signal was sent and a longer cable could be used. And there is not that much space in the BM7000!!
There will be far more space in the record deck you have - why not build it in there?
PS Look at the service manual of the 6500 - there is the accessory RIAA shown there - page 10 of the pdf