It likely won’t be implemented since there is no collision avoidance for all objects in between staves of grand staff instruments (only a few sort of objects - I believe it’s like a Newton three-body problem or some such). However, writing a graphical diagram in between staves is really not the accepted convention. Full graphical diagrams, which should be written very sparingly anyway, are generally at the top, sometimes at the bottom (Salzedo puts them at the bottom). Partial pedal changes (with letters) are to be written at the bottom, although in this case there are harpist who prefer them between staves, where you will have to deal with collision avoidance manually. I’ve seen the odd graphical diagram in the middle, but very rarely; it tends to collide with crossbeams, which are frequent in harp writing… There is no question that using “Diagram on top/Partials at the bottom” is easier in Dorico than other arrangements, but this really agrees with a whole whack of harp publications.