Have you tried the Raspberry Pi? It runs an open source version of the RISC OS. As for me, I’ve never had a BBC computer (I’m in my late 20s), but have tried to study their history.
But I have had and still use Macs And I happen to also play with Haiku. So I can help there. If your iMac is a Core Duo/32-bit (which could be a possibility, since it’s a 2006), your best bet is to use dd in Terminal to wipe the partition table on the hard drive, set up a legacy/Intel Partition Map or MBR table (Haiku includes DriveSetup for this), and install Haiku to disk from the DVD. The Mac should then go gray for a few seconds on next boot, but should boot up from the new install.
If it’s 64-bit (Core 2 Duo), I don’t think Macs supported 64-bit EFI until 2008, (so unless I’m wrong and Haiku has 32-bit efi), this won’t work. It’s the reason early 2008 Macs and earlier drop support after Mac OS X 10.7.5 (Lion). That said, your best bet would be to try a similar method as with 32-bit and write a legacy install to disk as the Mac has CSM (Compatibility Support Module or legacy boot support) built into its firmware.