Yeah, since there seems to be no way to “just” boot it I’ll open some tickets
oops you right, your mac is full 64 bits.
Btw i could boot haiku on MacPro5,1 which is closed as yours, using refind and disabling GFX acceleration (rx580).
Same variant 
but I had transferred Haiku to my iMac by imaging a snapshot of Haiku rather than using rEFInd or installing it normally
I tried this again, with opencore legacy macos installed haiku could boot fine with vesa, and it worked!
The I tried to install it, wiped the esp and macos… and now I can no longer boot it.
I’ve tried booting opencore so it tries to boot haiku, no dice. Tried refind chainloading and letting the firmware directly boot haiku.
In all cases the screen stays blank. Hope to get this working again but kind of out of ideas. MacOS patched is too slow to use on the machine.
I don’t know if helps, but using anything on Mac hardware, even with MacOS partition wiped and used for something else, I always keep 2 initial partitions: ESP and MacOS Recovery. If you put opencore to ESP, it still may need something (drivers?) from Recovery. Needless to say, the recovery partition contain some useful maintenance software for Mac.
In case I use a blank storage for Mac, I usually restore necessary partitions from Time Machine.