I can find nowhere in the NEC that says that you must bury inter-electrode bonding jumpers. In fact, 250.64(B) explicitly allows for running grounding electrode conductors (such as inter-electrode bonding jumpers) along construction or otherwise aboveground when suitably protected against physical damage, etc.:
Securing and Protection Against Physical Damage.
Where exposed, a grounding electrode conductor or its enclosure shall be
securely fastened to the surface on which it is carried. Grounding electrode
conductors shall be permitted to be installed on or through framing members. A
4 AWG or larger copper or aluminum grounding electrode conductor shall be
protected if exposed to physical damage. A 6 AWG grounding electrode conductor
that is free from exposure to physical damage shall be permitted to be run
along the surface of the building construction without metal covering or
protection if it is securely fastened to the construction; otherwise, it shall
be protected in rigid metal conduit (RMC), intermediate metal conduit (IMC),
rigid polyvinyl chloride conduit (PVC), reinforced thermosetting resin conduit
(RTRC), electrical metallic tubing (EMT), or cable armor. Grounding electrode
conductors smaller than 6 AWG shall be protected in RMC, IMC, PVC, RTRC, EMT,
or cable armor. Grounding electrode conductors and grounding electrode bonding
jumpers shall not be required to comply with 300.5.
Of course, you'll have to double-check with your local inspectors, though -- they may have some local requirement for burial of bonding jumpers.
Re-pulling through installed flex is a nightmare. If you do, keep in mind that new-style dimers require a neutral. -EMT fittings? I'm guessing there's no red bushing either.
Flexible conduit used as ground? - InterNACHI Inspection Forum:
"In general FMC or flexible metal conduit is limited to 6' in length when used as an EGC (equipment grounding conductor). Since this is an old installation it's likely grandfathered in, however by modern standards it's could be considered ungrounded even though all of the components of the system are metallic. Is the stuff in the photo actually FMC or is it old AC (BX) cable?"
250.118
(5) Listed flexible metal conduit meeting all the following conditions:
a. The conduit is terminated in listed fittings.
b. The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less.
c. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 1.8 m (6 ft). (does not include AC)
d. Where used to connect equipment where flexibility is necessary after installation, an equipment grounding conductor shall be installed.
Non-Armored Cable flexible runs more than 6 feet need a ground wire.
said by nunya, at dslreports.com, +1'ed by Speedy Petey:
Most BX is neither safe or acceptable for use as an equipment grounding conductor. Only BX which includes a bonding wire or bonding strip may be used as ground. Most "BX" style AC cable installed in the 30's-60's does not have the required strip.
Using BX as a ground is extremely dangerous, because it is not a low impedance path to ground. Rather than clearing a fault, the spiral jacket (especially in places where it might have some corrosion) will heat up like a toaster element.
I've witnessed this several times. The cable clamps in the original boxes also were not typically listed for use as a grounding electrode conductor.
Best Answer
Conduit (RMC, IMC or EMT) is an NEC-accepted ground path. So, if the wires are in steel conduit, no OTHER ground wire is required.
If that gives you concerns, you might find this link reporting research into the effectiveness of conduit as a ground relieves them.
Updated link to NEMA Technical Services Department Bulletin 97.