Well rectified AC or 240V will give 216V ~DC
IF 5V is wanted from such a high voltage then a duty of
5/216 = 2% which would be very hard to do (to derive a stable DC voltage)
Actually at work we looked into doing something like this. We need to derive a load of aux supplies: 28V, 15V 5V
from a bus of 270V the duty required was far too low (esp since our DC link can flucuate between 216 and 330V)
Our aim was to bring the voltage downto 50V, thus the duty would be alot higher (and thus controlable) the stability of such a rail would not be of too much concern since this crude 50V rail then fed more accurate 28V, 15V DC
C SMPS
In this instance I would look into finding a XFMR to step the voltage down to give you some more duty headroom. IF the challence is to do a SMPS, have a look into creating the DC (as you have) then AC chopping it at alot higher frequency, thus a smaller XFMR would be needed then tap down to something more manageble in the XFMR
otehrwise yes 2stage converter