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I have a couple of questions for you first:

With all due respect to Hopkinson et al., why are you interested in calculating such an antiquated metric?
Are you interested in how to do this with Radiance directly, or using some other tool that leverages Radiance?

~~I have a couple~~ The simple answer is "yes", Radiance can be used to compute DGI -- at least, it can provide high dynamic range images for inspection to derive DGI. This paper details the derivation of ~~questions for you first:~~the metric, and discusses its limitations as well.

Your workflow for using Ecotect would be to generate hemispherical fisheye view renderings of your design with Radiance, and apply the mathematical basis on them, or (easier) use evalglare to interrogate the HDR images and give you DGI (evalglare will also give you DGP and a host of other metrics as well).
With all due respect
BTW, Evalglare is most likely going to ~~Hopkinson et al., why are you interested~~ be part of the standard Radiance distribution in ~~calculating such an antiquated metric?~~
the near future. In the meantime, I think the only link to it is here.
~~Are you interested in how to do this with Radiance directly, or using some other tool that leverages Radiance?~~