I have a SSD drive on my MacBook Pro and I'm constantly running on out of disk space because I need to install and run multiple virtual machines (Linux, Windows etc) on my Mac. Is there a way to mount a writable disk image that is also compressed? I'm hoping to use the disk image to store the VMs on. I don't mind taking the speed hit.
This doesn't answer the question that you asked, but it might address the problem that you have.
VMware Fusion 6 Professional allows for linked clones, which are copies of a VM that don't duplicate the entire contents of the VM. If you've got lots of VMs which are based on the same OS, you can save yourself considerable disk space with them.
To create a linked clone from the current state of a VM, select your VM in the VM library, then click "Virtual Machine" and select "Create Linked Clone". You can do the same from a snapshot of a VM if you prefer.
The biggest thing to watch out for when cloning a VM is to ensure that you don't have an IP address conflict, so change the IP address of your clone if you're using static IPs for VMs. Linked clones should save some disk space while avoiding the performance hit of ZFS.
I ended up going with MacZFS.
I tried booting a Windows XP VM off the ZFS file system and it worked. ZFS reports that it achieved a compression ratio of about 1.32x (which means my 10GB VM takes up about 6.6GB on disk). Not sure how well VMWare will play with ZFS's compression under high load, but it seems to working well so far.