All you really need is some wire and a clamp on ammeter that is capable of measuring DC.
You can get a DMM with a current clamp built in. Something like the
Fluke 336 wouldn't be bad. It would give you a good general purpose DMM with AC and DC clamp built in. For DC current it's accuracy is spec'd at 2% +/- 3 counts. (I interpret 3 counts to mean 0.3 Amps, but it would be best to check with Fluke.)
A definitely cheaper option would be to get something like this
Fluke 36 clampmeter on Ebay. I don't know the specs though.
You can also get clamp on probes that are meant to connect to a standard DMM such as the
Fluke i410. (3.5% of reading +/- 0.5 Amps) The error of the DMM you plug it into gets added on to the accuracy of the clamp itself so the total accuracy would be substantially less, but you might find these much cheaper.
In case you hadn't noticed, I haven't mentioned anything other than Fluke products. I'm not aware of ANY other sources worth looking at for this kind of stuff.
If you (pH or jaydee) get a clamp on, I can characterize its error over a span from say 10A to 100A which should allow accuracy substantially better than the manufacturer's spec to be achieved. (Although, how long, and under what conditions such characterization holds true is open to question.)
The next step is to modify the motherboard. (The following is a lot of words, and pictures might be better, but look through this, and let me know if anything is unclear and I'll try to draw a picture to explain it.)
There will be several inductors in the Vcore regulator circuit which the current to the CPU flows through. (Two to four for mobo's made in the last couple years.) These are the little toroids (doughnuts) with wire wrapped through them near the CPU.
The basic idea is to connect the output side of these inductors to wires which go through the clamp and then back to the holes in the mobo that the inductor's output lead originally went to.
One end of the inductor will connect to a MOSFET (or two) and the other will connect to a capacitor (or two). The output side of the inductor is the side connected to the capacitor(s). Unfortunately you can't just use an ohmmeter to determine which end of the inductor is which. (The resistance of the inductor is so low that it's effectively a short to any handheld ohmmeter.) It's likely that you can visually look at the board and determine which side of the inductor connects to the cap(s) and which to the MOSFET(s). If not, you will have to unsolder at least one lead of the inductor, and use an Ohmmeter to determine which inductor lead is which.
Once the output lead of the inductors are identified, they need to be desoldered from the circuit board. Then you need to attach a fairly heavy wire to the lifted lead of the inductor. Ideally you would use the thickest solid copper wire that will fit into the hole in the motherboard that the inductor lead was pulled out of. (I'd say 16 to 18 gauge at a guess) Start with about a 1 foot wire for each inductor. Wrap one end of this wire around the lifted inductor lead and solder the connection. When you've got the wires soldered to the inductors, bring them all together so that you can get the clamp around all of them simultaneously. Then cut them down to where they are just long enough so that the loose ends of the wires can be soldered into the holes the inductors were pulled out of. (It's note likely to matter whether the wires go back into the hole for the inductor they are soldered to, but just in case, make sure to match the wire from each inductor with the correct hole in the mobo.)
Ideally, where the wires are to pass through the clamp, they would be close together, and pass through the exact center of the clamp. You want to avoid having any of these wires running near the 'gaps' where the jaws of the clamp come together. There is the obvious 'gap' where the clamp opens, and the less obvious gap at or near the hinge point of the clamp. One of the major sources of uncertainty in clamp-on measurements, is the location of the wire with respect to the 'gaps'.
You've got mutually exclusive goals here. Keep the added wire as short as possible, yet make it long enough that you have a nice big loop that you can easily clamp your meter to while keeping all the wires away from the 'gaps' in the clamp. Use your best judgement. The manufacturer's spec includes uncertainty for where the wire is located wrt to the gaps, so you should get better accuracy than spec. (But who know how much better?)
Once you've figured out how you want to route the wires, solder the loose ends into the holes left in the motherboard and you are ready to go.
Everything should run fine regardless of whether the clamp is used or not. The Vcore regulator will be somewhat less efficient due to the added wire, and it will radiate more EMI, but neither of these issues is likely to have a substantial impact. (In pH's case the need for twisted leads from the CPU diode to the diode reader will be a bit greater, but hopefully the leads were already twisted anyway.)
It would probably be easiest to work with magnet wire, (The 'enameled' wire that is used to wind motors and transformers.) but I'm not sure where you could get a small quantity. If you find out the ID of the holes in the mobo that the inductors solder into, I can send you the gauge of wire that fits.