Here's another vote for GPT.
Obviously you know your customers, so the following suggestion is just a "what if"...
Your customers might reflect your feature set and not necessarily what the iSCSI market wants to/could grow into. Again, I think you stand uniquely poised to fill a vacuum. Ethernet is only growing, and this make iSCSI a shoe-in. Two 1.5TB Seagates in RAID 0, or three in RAID 5 costs only a couple times as much as your license. This means providing access to large storage would open doors in your price band. Small businesses, hightech media hogs and one-man-bands *can* afford your software, not just freebie downloaders. For example, I would readily pay $200 for the following feature set:
x64 (not just for kicks - I do heavy media work and use VMs)
GPT (I have "large" drive sets for backup for local and my other workstation)
loopback or 64-bit vmdk handling
Joseph