Seems like a gap in the market if someone steps in with offline sync (even just one-way) and the ability to securely access files from a next-day delivered USB stick.
We've done that for special case scenarios. The last review tool we used was in a mixed Linux/Windows environment and we had a client in jail awaiting trial so we built a laptop to run everything and our PM visited him weekly to update his copy of the DB locally (no internet access in the pen). The biggest sticking point is that it's not uncommon to get to 4-6 TB on one of our databases and it takes a bunch of processors and memory to run the software, but I smell a development opportunity now that you mention it.
One, you can get an external 6 TB hard drive for like $100. At attorney levels of money, that's entirely reasonable.
Two, clearly in the pre-computer world, attorneys were not reviewing 6 terabytes of actual data for discovery. What makes it so big? Is it that you have scanned PDFs in image format (such that it's a reasonable number of pages printed out), or is it that you have new types of data like electronic records that simply wouldn't have been picked up in the old days?