debian and Ubuntu have a package called "hfsprogs", which can be used to create HFS+ disk images:
apt-get install hfsprogs
Initialized disk.hfsplus as a 100 MB HFS Plus volume
disk.hfsplus: Macintosh HFS Extended version 4 data last mounted by: '10.0', created: Mon Mar 23 18:32:16 2009, last modified: Mon Mar 23 18:32:16 2009, last checked: Mon Mar 23 18:32:16 2009, block size: 4096, number of blocks: 25600, free blocks: 25197
sudo mkdir /media/hfsplus
sudo mount disk.hfsplus /media/hfsplus/ -o loop
Now we can copy something there
(How) can we use this disk image with qemu?
If we want to make this bootable with a boot loader, we need to have a partition table inside the disk image!
Probably we need to do this along these lines: