Metadata controller options?

We're a small studio (3 users, about a dozen workstations), that specializes in film scanning, color correction, restoration. We're dealing with very big files - a combo of large image sequences and large containerized files (ProRes). We have a 40GbE network that has been up and running for a few years, and all machines are connected over SMB. Each workstation has a large local RAID, so we've been able to limp along by reading stuff over the network from one shared local RAID, doing something with the files, and writing to that workstation's local RAID. But this has obvious limitations, so over the past few months, I built an iSCSI SAN. It has two 60TB hardware RAID6 pools, which we've divided up into 12-16TB iSCSI targets.

The hardware has been up and running for a while, but we're using it the way we were local drives: A workstation will mount an iSCSI target, and then use SMB to share that with others that may need occasional access. This kind of works, but what I want is to have multiple machines with read access on a target that one machine has read/write access on. SMB is a performance bottleneck, so I want to be able to connect directly over iSCSI.

So I am looking for a metadata controller for iSCSI on a 40GbE network, software only, that will work with our existing hardware installation. So far I've found a grand total of two options:

TigerStore (used to be MetaSAN) - about $1000/server but price is tied to storage size. More space = more costs, so we're probably looking at $2k to start, $3k by the end of the year. This seems an arbitrary way to price, and it rubs me the wrong way. Also, I haven't heard from them after trying to contact them a few weeks ago with questions, so that's not encouraging. 

iSANmp (Storage Network Solutions) - not a metadata server, but a peer-to-peer system that requires the software be installed on each workstation, at $200/seat. Probably in the $2500 range to set this up. Also, each machine uses a USB dongle, so adding new seats is a little bit of a pain.

*all* I want is a software solution. I don't need a turnkey system. We can build the server or use one of our existing servers for this purpose. I dont care if the metadata controller is running on Windows or Linux, but we do need support on the workstation side for Mac, Windows 7 and Linux (CentOS 6 and 7).

Is an inexpensive software-only solution to this problem a pipe dream? I'm hoping to make a list of stuff to look at at NAB so any suggestions are welcome.

Thanks!