Comment # 17 on bug 908454 from Ruediger Oertel

file only tells the type, not the version we are after.
for reference:
http://morgue.openinkpot.org/wiki/Documentation/ZImageFormat

in short:
objdump -EL -b binary -D -m armv5t zImage-3.16.7-11-default | grep 8b1f | head
-2
    4878:    00088b1f     andeq    r8, r8, pc, lsl fp
    5220:    8b1ffd06     blhi    0x804640

4878 hex = 18552 dec

dd if=zImage-3.16.7-11-default of=piggy.gz bs=1 skip=18552

# get_kernel_version piggy.gz 
3.16.7-11-default

no idea yet how to get a simple implementation for this without all the ELF
magic