Isolating DC/DC converters to suppress undesired current return paths
You have stated that the ground loop is a problem, but you haven't stated why it is a problem. Typically a ground path like this will cause two problems:

The ground will bounce from common mode noise.
The loop of the two grounds will pick up magnetic noise.

The biggest issue is currents should not return on the chassis because it creates saftey hazards during faults and hot plugging (if power connects first the current can go through the chassis, at 12 volts this is not dangerous to a person but it could be to your electronics).

What is the name of this concept? I've done some googling on this
subject but seem to lack the appropriate keywords.

Power isolation

Can anybody point me to some literature where such issues are
discussed? I've done some reading in Lock / Joffe's Grounds for
Grounding, however, I cannot find the topology corresponding to my
problem in their text and a discussion in terms in currents as carried
out in the first figure seems not to be available.

You probably won't find that specific diagram, but if you want to read up on how to eliminate noise sources and ground loops or other challenging noise situations Electromagnetic Compatibility Engineering is for you.

Source: http://www.hottconsultants.com/EMCE_book_files/emce_book.html

Is this a valid approach for suppressing the undesired return current
indicated in the first figure in green?

Isolation is a good way to stop common mode noise from return currents, a DC to DC isolator wont really care about the a changing ground voltage, and it will regulate the voltage to a device accordingly as long as the input voltage to the isolate is within it's spec.
The ground then carries very little current (currents that might be capacitivly coupled), and no changing current from the load in the device, because the current from the load returns to the DC DC converter.
There are many of off the shelf isolators available from distributors or power supply manufacturers that have been tested and meet FCC requirements (although these do not apply to many vehicles)