There is nothing wrong with using an iso transformer PROVIDED YOU KNOW WHAT YOU ARE DOING.
There are even fully isolated bnc cables and probes for such circumstances. There simply are situations where you can't go without. Take an H bridge in a motor drive: 400 volts dc( I'm talking frequency drive ).
Your task is to visualize the gate drive voltage of the top transistor.... You cant do it because the scope does not have enough dynamic range to show a 2 volt signal superimposed on a 400 volt offset.
The solution ? There are two : use a high voltage differential probe or float the scope...
Tektronix makes a scope that has separated and isolated inputs. 4 channels. The input bnc is not connected to chassis ground. Each input channel has its own psu digitzer and memory and communicates over an optolink with the main processor.
Back in the day,Tektronix made plugins for the 545,etc,which had an offset control,so you could look at quite small signals superimposed on,from memory,up to around 400 volts dc.
This was a much more common requirement in the days of tube equipment,when anode voltages were anything from 200 volts upward,depending upon the application.
I assume this would work in the application you referred to.
It is a shame that they have stopped making things like that,although the special isolated design you described does the same job.
What I can't quite understand is the OP's requirement for a floating device for automotive use.
All the automotive waveforms I have ever seen were expressed w.r.t. the car chassis.
It is fairly easy--if you stick your earth clip on a car body,engine block,etc.you can use the probe to look at waveforms with no chance of damage to your 'scope.
In any case,most circuitry in cars is protected by fuses,so you will probably blow the fuse & protect the 'scope.
Even if you are careless enough to put the earth lead on the "hot" side,the 'scope body still has to be touching the chassis to cause problems---stick a newspaper under it,& you have a "floating" instrument
as far as the car supply knows.
Under normal conditions the car "earth" & the mains earth have no connection.
*Looking at
mains voltages direct is another story altogether,&
special precautions need to be taken.
*If you are running your Oscilloscope off the battery,using an inverter which connects the earth pin of the output socket back to the car chassis,(which is fairly uncommon) the 'scope body will be at the same potential as the car body/chassis,so it would be important to get the probe earth lead & probe tip the right way around.
If you have a DMM,you can check this at the outset.
If you are paranoid,you can also use the DMM to check any points you wish to probe with your 'scope,