whats wrong with using the lt3080? you can get a nice high output current with them. a microohm meter needs a beefy pulse. the ltl308x variants can go as high as 3Amps... idk about slew rate and stability for capacitance but it should not matter.
now, i wonder what the drift specs are.. but in pulse mode with low duty cycle you might have found a perfect solution. the only problem is at this point you have 2 voltage references, in the LT3080 and in your control circuit. this may or may not be good enough for your application, i suspect that this will be a fairly solid solution from something that is put together from only a few parts with minimal design effort though.
use a EXTERNAL digital to analog converter with reference to control the lt3080 in a precision application.... i don't see why you would use a god damn pwm from a pic to do this (so ghetto
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ikea solution!
don't do this for nice test equipment ). its great if you want a interview prop for some kind of bullshit cheap ass company only
(p.s. you wont want to work there for long)
if you want to be l33t figure out how to use 2 of them so you can send a negative and positive pulse, then find the difference in the measurement (offset error/noise elimination). like, use a ADC + multiplexer to measure the exact voltage of each lt3080, set pos/neg rails appropriately, then send pulses through your load using mercury wetted relays, then tune your sampling correctly.
if you build it well this will be a absolute beast