I'm guessing one of the major barriers to split right shift isn't just a design decision, it's availability of compatible keycaps.
Split right can't be accommodated due to the post/standoff that connects from the case to the plate. It interferes with where you would insert the switch. Having tested and played around with the proto, I'm pretty happy with the amount of flex and give the board has and I think moving the mounting point a column up or down, or even to the middle would shift the stiffness of the alpha section significantly.
Cap availability was also a big thing with Lunar mk1. Either salvage a second board for a single cap or have a resin cast one.
That said, I'd really enjoy having split right shift for hhkb arrows... I'll probably just map shift to fn and still throw hhkb arrows under there.