A MOSFET + a few resistors is *NOT* a regulator - it can be used as part of a regulator circuit but without other semiconductors it will NOT stabilise the output voltage.
There are regulators with a higher input voltage than an ordinary LM317, e.g the LM317HV which is rated for up to 60V input. However in that circuit it will blow at switch on if the transformer output voltage exceeds 42V RMS, and as there is only 12V headroom between 60V and the desired 48V out, and the LM317HV will drop out at a bit under 3V, that means you've only got 9V headroom for ripple, transformer regulation and tolerances and mains voltage variations. That isn't anywhere near enough for reliable operation.