Primary/Standby is certainly not the only way to achieve fast restore times and minimize lost data. But it is a very elegant way since it is built into FMS. And it is cross-platform.
One thing you can not do with RAID-via-Target-Mode is very effectively switch roles without cutting of clients. Or temporarily break the connection and resume it later.
I don't think you should try to make Primary/Standby and RAID-via-Target-Mode work together. You need to let the Primary FMS box be in control of pushing updates to the standby. The main thing with Primary/Standby is that they are two different machines.
I see what you are saying about being built in to FMS and not losing clients being a big deal.
RAIS TDM uses two machines as well. Some people have been using it with FMS for a while now. Not as graceful as Standby.
I look forward to some highlights from your presentation after Devcon is over.