I'm talking about Adum5000 chips, they're digital power isolators which can do up to 5v at 100mA (500mW) but the efficiency is poor ... 34-35% :
See page 5 , 34% max at 100mA output : http://www.analog.com/media/en/technical-documentation/data-sheets/ADuM5000.pdf
So he could have two in parallel and have 5v 600mA in , 5v 200mA out (1000mW), and then a boost regulator could produce 40v at 20mA (800mW) with around 85% efficiency maybe.
So yeah, it would be very tiny, low height, but expensive and low efficiency. You'd save some money on a potentially custom high frequency transformer for isolation purposes, but you'd spend the money on the adum chips and you'd risk not being able to buy them...
Ok, noted for potential future projects where a flat size is needed.
The 6$-15$ isolated DC-DC converters are already isolated, no xformer and you get the 48v, or any voltage from 3.3,5,6,10,12,15,24,30,48, @ 1watt,2watt,3watt(getting expensive),5w(getting very expensive) directly in 1 step with around 80% efficiency, but, it's a small brick shaped device.
I don't know why people complain about noise, I usually place a Polymer Aluminum low ESR cap at the input and output, an they become clean as a whistle. Sure, just placing just the recommended electrolytic cap in the data sheet leaves you with the ripple noise listed in the data sheet. But, the data sheet doesn't say you cannot use caps with around 2 orders of magnitude smaller ESR to make to output clean as a battery.