I installed Haiku on HDD of iMac.
Nothing special is necessary in my case. You can consult the official guide to install Haiku. In my case I had to manually copy haiku efi hootloader to EFI partition using these instructions. AFAIK it was fixed still then and Installer does it automatically. The only unpleasant issue was to eject the installation DVD after installation. I did it by pressing Alt key at iMac start to enter choose boot media, and then pressed Eject key from keyboard (only available in mac keyboards).
You may want to look at this thread for running Haiku on Macs.