By "USEC" do you mean the United States Electrical Code? If so, I've
never heard it called that -- it is the NEC, "National Electric Code".
The NEC does (I am pretty sure, as of as recently as a year ago or so),
still allow source-light-switch connections. Nothing wrong with it at
all, and a perfectly fine way to do things. You are right that the hot
has to go straight through the light box to the switch. The reason is
that when the switch is turned off, there should be no HOT connection
to the light bulb at all, so you can change it and whatever without
risk of shock.
But really, the OP should have no trouble. Just look at the existing
light. Whatever two wires connect to the light bulb fixture itself are
the exact two wires you want to hook the new fixture to. Should be
simple. If it is up to code, it should be a white and a black with
nothing else in the box (source-switch-light), or a white and a second
white relabelled as black (source-light-switch) with two other blacks
in the box.