Cool, Commodore128. Now if I could only get it to work with Android 4.2. I had to revert to 4.1.2, since Directory Bind and apps are stymied by 4.2's emulated directory structure.
In 4.2, I could get Directory Bind to bind the true source directory path (/mnt/shell/emulated/0/usbStorage/sdb1/xxx) to the fused/symlinked/emulated target directory path (/sdcard/xxx) on second mount (sdb1, NOT sda1), but apps didn't see their data. I think it would work in 4.2 if Directory Bind would accept the true target directory path, but since Directory Bind is no longer supported by its developer, that isn't likely to happen.
I hear there's a hacker warez called Lucky Patcher that supports directory binding. Until I check it out, I'm sticking with 4.1.2 and Directory Bind.