I don’t think Fusion supports Firewire port mapping anyway - but even if it did, you would only be able to hook the storage to one VM directly or share it from the OS X Server host over standard networking or Shared Folders in Fusion.
iSCSI is really the only way to go.
The other issue is that you can’t configure any vSwitches in Fusion - so even if you had 2 NIC’s (1 for storage 1 for other traffic on the host OS X Server) I don’t think you can dedicate those in Fusion, as you can in vSphere, for example.
You would still gain some performance by having the storage dedicated on the host though.
Arguably the best way to go (in my opinion) with a single host restricted storage device like the DroboPro is vSphere. Though breaking the Apple EULA, you ‘can’ run OS X Server on ESX/vSphere too. That said, I have VMware NFR licenses with my job so I get all the nice vCenter and Data Recovery options free - alright for some. It just so happens that it works perfectly for this type of device. The Elite would be different - if only I could have afforded one!
If you go with your idea, I would highly recommend having your OS X and VM storage disk(s) mirrored. The next issue I had was how to get my VM’s highly available as I was now dependent on them to access the Drobo - you would be dependent on the OS X Server for access to the storage and the VM’s as your media server etc .
What happens if the OS X server OS disk dies or the host itself dies? Or your VM’s don’t work for some reason? Obviously your Time Machine on the Drobo is fine, but without your host, the backup is useless. And you can’t restore an OS X server (or client) from a non-time capsule network TM backup - you have to do it after the restore process of OS X. Your down time may not be an issue, so this may not matter. It’s more important for me, perhaps.
I use VMware Data Recovery and DroboCopy to backup the VM’s. In the event of a major issue, I’d have to do some work, but essentially if I mess up a VM in any way, I can recover very quickly. I also have the ESX host on mirrored drives - it’s just an HP DC7900 business workstation - works nicely.
Hope this is helpful in some way shape or form. Best of luck with everything.