Mark,
Thanks for the clarification. If I am not wrong what you need is a location to store ISO's that can be shared between Windows / Linux / ESXi correct?
Assuming this is correct then the best option is a virtual machine running linux or some type of freeNas Open filer type appliance. If you use linux you could present the ISO's to Linux and ESXi as NFS and present the same mount point via Samba (CIFS) to Windows. It would work great... I would suggest that you present it readonly to Vmware to avoid things like datastore heart beating on it thou.
Just my two cents.
I'll see if I can do a write up of the process in the near future if your not familiar with it.
Thanks,
J