debian and Ubuntu have a package called "hfsprogs", which can be used to create HFS+ disk images:

apt-get install hfsprogs

mkfs.hfsplus disk.hfsplus
Initialized disk.hfsplus as a 100 MB HFS Plus volume

file disk.hfsplus
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:
http://wiki.eeeuser.com/howtocustomrestoreimage:pt2mkcustomimage