Aside from sounding like it might actually be practical, it also sounds like fun. What if the remote node (or the local node, if it is allowed) is an NFS volume, which is itself under DRBD control? The devil in me wonders about a chain of these beasties, especially if you can see or hear drive activity, activating one after the other, fractions of a second apart.
How much delay is there from node protocol? If you had an infinitely fast network, how much difference is there among the various Protocols and compared to a standard local filesystem not under DRBD control?
Is it possible to choose whether a disk is under DRBD control? COuld you run benchmarks both ways with just a simple umount / mount between?