> meter to the pri, then put some dc into the sec
True and a useful simplification. I'd assumed it would be useful to have a gizmo that could measure just one coil, for chokes, for wacky-Z transfos, or getting a take on a failed transformer.
Also, shorting any secondary tells you the leakage inductance, often a handy number to know. (Though this rarely depends much on DC current.)
Either way, the hard part is making a DC current with very high AC impedance. Big resistor from a high DC voltage works, with small error that can be subtracted away. But this tends to need a power supply as big as the final project. That may be handy if you already started the project, might be awkward when just going through the junk bin.
> since the secondary is usually spaced further from the core, is it really "all the same" as far as the core is concerned?
Yes.
Feed a strong AC voltage to a low-volt winding on an open transformer, one with a little space between the outside of the coil build and the core. Get a yard of thin wire. Pass it through the space once, measure the induced voltage. Pass it through the space and almost to the other space, a half-turn. You get the same voltage. Pass through the other space, the voltage doubles.
It isn't the number of turns, or the location in the build. It is the number of passes through the core (two per "turn" on common US E-I cores).
Actually, location and proximity to other winding must make some difference, but for iron-core 20Hz 50Hz 60Hz coils, the effect is less than 0.001 or 0.1%. 999 flux lines flow through the core, and thus induce all other windings, for every 1 flux line that flows through air.