2007 iMacs likely have 32bit EFI, in particular a proprietary firmware which implements EFI 1.10 specs and so called fat EFI binaries support.
I don’t have any 2007 model to check but the issue is likely that AFAIK it’s not possible to boot Haiku 64bit out of a 32bit EFI. I think @davidkaroly is working on a hybrid bootloader. More on this here.
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