I can think of one other way which is more likely to work.
Put a small shunt resistor (again, a short wire with Kelvin connections) in series with each of the switching inductors of the Vcore regulators.
Attach a high bandwidth differential amplifier to each of these Kelvin connections to get a signal that gives decent resolution on a scope. Look at all three or four of the amplified signals on a scope, and gauge the current from these signals. These signals are going to be very noisy so it's going to take some interpretation to know what is relevant.
Not going to be that accurate...
Okay, yet another method. Most practical of all.
Just remove the Vcore regulators. Hook a seriously beefy bench supply up in their place. Measure the current coming out of the bench supply. The biggest problem with this (other than finding the supply) is getting the bench supply turned on at the right time. I have no idea what the power sequencing requirements of a PC motherboard are. Probably have to rig something up to get the bench supply switched in at the right time. Hopefully, not too much hardware gets destroyed in the process of getting it right.
[I was supposed to be working on a spreadsheet tonight. It will have to wait.]
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