How would you attach them? Just glue, or mortise and tenon, or?
Would they be pre-drilled/slotted?
From a Japanese perspective, wa handles are often sold separately and blades can be removed and installed into other handles... but this gives that narrow tang/somewhat cosmetically imperfect fit at the steel/bolster joint. If you do it this way the separate handle has to be pretty strong without a blade though it, i.e. not just epoxy but mortise/tenon or other mechanical means of reinforcing the joint in addition to glue, particularly with woods such as coco and bw that are so oily they may adhere with less strength. The "Dabney dovetail" is a cool example although in that case I believe the bolster front is fit close to the specific blade.
From a western perspective, often our take is to make the wa handle monolithic with the blade- i.e. stays on there unless destroyed off. This gives a cosmetically perfect tang/bolster joint, and since the tang stays in there from start to finish, it is a suitable reinforcement for the bolster joint. To do this though, the main point of the bolster as in other types of handles is to serve as a cosmetic trim for the tang cavity. One drills and broaches the handle block so the tang fits in, but carefully slots the front of the bolster to a "guard" type tight slot. So, it really has to be fit to the specific knife, during the build process as a whole.
All this to say, from a western approach I don't think it makes much sense to try to glue the bolster on before fitting the handle to the tang. From an eastern approach, you can do that- but at that point you're pretty much in the "wa handle game" outright, and would be more or less making finished reinforced handles and selling those, as many do.
Another thing- I don't like to use natural woods, even blackwood or coco, as a bolster on a kitchen knife. Even stabilized woods and some types of synthetics will move a bit or shrink at times. With a bolster fit close around steel, the unyielding nature of the steel as well as the somewhat more prolonged exposure to moisture at this end of the handle can promote cracking if the bolster material shrinks even a little. I've seen some paper micarta such as paperstone shrink and crack here. G10 might be the most resilient, or quality micartas. I'll put a coco block or stab burl etc behind such a bolster, as the added epoxy in the joint makes things just a little more forgiving.
