Anybody know of a SMT regulator which can handle 45V input, 5V/50mA output?
    40V * 50mA = 2W - an awful lot for a tiny PCB to handle.

That's well within the range of telco-oriented dc/dc converters (which have
input voltage of typically 36-72V), although those tend to start with a
particular package size and cram as many watts as possible into them, rather
than picking a number of watts and making the package as small as possible.
For example, in about the size of a 24pin DIP, Datel has a 3W converter.

I suspect you had something much smaller in mind...

BillW