As bug #132120 suggests, I've verified that just replacing the busybox binary (both as /bin/busybox and /bin/sh on the initramfs) with an older, working version is sufficient to fix the problem.
Replacing the failing calls to mount with explicit calls to /bin/mount - thereby invoking the klibc version of mount, rather than the busybox version of mount - also fixes the problem (well as long as the -n flag to mount is also removed, since it is not recognized by the klibc version).
Thus, it looks like there's definitely a bug in busybox - although I don't know if initramfs-tools was heading towards removing use of busybox in favour of the klibc binaries, which might make it moot.
As bug #132120 suggests, I've verified that just replacing the busybox binary (both as /bin/busybox and /bin/sh on the initramfs) with an older, working version is sufficient to fix the problem.
Replacing the failing calls to mount with explicit calls to /bin/mount - thereby invoking the klibc version of mount, rather than the busybox version of mount - also fixes the problem (well as long as the -n flag to mount is also removed, since it is not recognized by the klibc version).
Thus, it looks like there's definitely a bug in busybox - although I don't know if initramfs-tools was heading towards removing use of busybox in favour of the klibc binaries, which might make it moot.