I doubt that microprisms or split image would fit your needs, if you will REALLY get into the macro field. Mid format lenses are never impressively fast on their own, and you've to add to that the fact that the light will be further reduced by the bellows extension or the use of additional rings. The prisms will turn black and thus useless to help focusing.
That's why there is the "cross-hair" screen which is specific for macro work (or for super-long tele lenses, which are dark for other reasons). The central portion of this screen is transparent (not even grounded) with a hair-thin "cross" engraved. You have to focus the aerial image (that is the image will not be projected on the ground glass, as it is transparent, but will float on space) so that it appears in focus together with this hair-cross. When you see both the cross and the aerial image in focus, then the focus setting is correct.
I don't remember the number right now, but If you look in the RB user's manual, you'll see that this type of glass is available. This kind of glass, as you see, is not of immediate use and needs practice, but is the only one of any help that I know (over the pure ground glass) in low aperture/light conditions.