From my understanding, they are two totally separate indicators.

The L5 Nerve sign relates to an abnormal lumbar nerve whereas the Chapman's sign is a signal on an ECG indicating a specific cardiac issue.

"C"

I found this article:

http://theassr.org/abstract/l5-nerve-sign-applying-the-lessons-of-periph...

Thanks. That abstract seems very useful.

I have discussed this with a neurosurgeon who says that the L5 Nerve Sign is recently described by a radiologist. The observation is largely anectdotal, without a lot of scientific proof so far.I can not find any publications in mainstream medical publications.
In any group of back pain patients it doesn't seem to be very common. But if you find it, it probably means that there is abnormal pathology in the L5 nerve and it seems to imply permanent nerve damage that may persist and be the cause of failed back syndrome. Rarely it can mean the presence of neoplasm like schwannoma, or inflammatory neuritis.