Hi Kevin,
Yes, it is possible to have a calibration mechanism that compensates for resistor errors in an R-2R design. If done correctly it can yield good results initially. The difficulty here is that naturally some of the current sources in the DAC will be turned on/off more than others given that a current source is correlated to a particular bit in the digital audio word, which in turn causes uneven aging of (among other things) the resistors in the current sources. This will cause the resistance to drift out from the original compensation over time, with linear distortion occurring as a result.
If you are operating the Ring DAC correctly (or any thermometer DAC for that matter), over time all of the current sources (and therefore the resistors) will be used the same amount. They will therefore age in a uniform way. We do account for resistors aging in the Ring DAC on the software side, made possible by the uniform aging of components.