Elsewhere on this forum is a thread for piezo current measurement for injector diagnostics. Now, strictly speaking, this is not quite what you were driving at in your post (I think) because this is trying to use an electrical charge characteristic to measure an electrical failure (piezo stack failure) rather than pure and simple hydraulic sticking. It does work to some extent, but you have to be careful.
As alluded-to in your post, you are better looking at cylinder contribution (through flywheel velocity) for a low/high injection quantity on one or more cylinders, however your are more likely to find that the scan-tool data for the ECU making that same correction is much more useful that whatever you can derive from the Pico (a rare thing, but for a good reason), so unless you can switch-off the cylinder balancing function in the diesel controller, then you are better using the scan-tool information. The Pico information for a "low" or "high" cylinder would take a huge amount of processing to be as good as most diesel controllers, but you would also have to know how the balancing function was switched against each engine operational domain (early Bosch functions only operate near or at idle, for example) and whether the data is kept from one drive cycle to the next, how it is used (if at all) at higher engine speeds and loads, etc. etc.
Also in this forum are a few threads on using rail-pressure to establish a sticking needle. Yes, you can do this, but you have to be really careful to not paint every engine with the same brush! How the fuel rail is laid-out can make a huge difference to the way that the RPS signal will fluctuate from cylinder to cylinder, even on a good engine, and also the way that the pump is set to compensate the fluctuations can make a big difference. So you could end-up with an answer for one engine type, which is useless for another.
All the best with your work!