I am not positive about Boston grands, but I would guess that they are the same as Kawai.
The Kawai caster socket is a cast brass tapered design with a flange on the bottom (4 screws passing up into the leg), and the caster stem is pinned into the caster socket and is not removeable (at least not easily). So usually when the piano is put on a dolly the caster sockets are removed along with the casters.
Generic casters will not fit in this leg.
PianoForte supply in Vancouver used to carry German made casters with sockets to fit Kawai legs. They were excellent casters, and available with a rubber 'tire' to protect floors. They were also very expensive. You could email Jurgen to see if he has information about them, or you could contact Erwin Piano Supply to see if they are stocking these casters. They really are very well made.
But first you will need to check the bottom of the Boston legs, or call someone familiar with the Boston pianos to see if they have standard Kawai style casters and sockets.
Don Mannino RPT