I'm looking for information on bare 25 gauge (~0.5mm diameter) cable being
fastened to cars being transported from Japan specifically fastened
underneath the hood and with the possibility of extending out along
the body.  I believe that this could be the case for example to ground
the cars.

Do you want to buy some 0.5 mm cable?

I am looking only for information (an account, preferably) on its use
as a ground wire while transporting cars by boat or rail.  I am going
on the possibility that an extended amount of time on a boat or train
can allow a dangerously high charge to build up in the car as it will
function like a capacitor... first person that steps in could receive
a big shock.  So what can be done is to connect a conducting wire from
the car's frame to the boat's frame to eliminate potential
differences... But is this done in practice?

Hi, i am a professional electrostatic painter, the grouding cable you
speak of is infact used when shipping large industrial parts by sea. i
have seen it in use on the following

:large machinery parts (grounded to exterior)
:service bodies for pickup trucks (see above)
:complete mining equipment ie:vehicles (grounded through charging system)

in a related bit of info, the 25 gauge unshielded wire is the same as
we use in my trade for grounding large piece of equipment as our
system uses a 120kv dc charge to apply the product.

hope this helps you in some way.

-Bota