Just had a quick look on the SRM website and it got me thinking on a few things hopefully someone here may be able to answer…
One of the pics showed that the strain gauges go inside the powermeter in the four ‘quadrant’ sections. I gather that this means the pro version has one gauge in each quadrant, the science version has two per and the amateur version only has a single gauge in two of the quadrants. Anyway if the gauges are in the powermeter why could you not use your own chain rings and crank arms with the units? I realise that typically the right crank is built into the ‘crank housing’ but if you can buy spare crank arms on the srm website does that imply that both arms can be replaced at different lengths ie you could therefore use your own crank arms?
Edit: Ok had another look and realised the right crank arm is screwed into this powermeter, hence normal arms cannot be fitted this way.
You only have to use the right SRM crank arm. The left crank arm can be anything you want it to be. I just changed my right crank arm length and these are very precisely machined/forged to fit into the crank assembly. I have no knowledge of what the DuraAce or Cannondale one piece versions look like. One can use whatever 130mm 5 bolt chainrings one wishes as well. The powermeter on the road pro version and road amateur versions the holds the right crank arm is water proof and a custom assembly holding the transmitter/strain gauges/battery/cadence magnet - good luck getting any help from SRM if you wished to take the guts out and putting it on your own crankset.
Even if you drilled out a right crankarm from an existing crank one would have to machine it to just the right size. It would not be worth the effort in money and time and would probably be weaker than the forged/machined part from SRM.
One more thing. The Campy version requires the Campy bottom bracket for a good chainline so you have to use a Campy compatible left arm or sometimes a non Campy will work, it just depends.