I'm not sure how we'd go about it in a way that makes sense. Pretty much the entire point of doing box sets is that it's a physical product, and as you pointed out, the only difference between that and buying the content individually is the illustration on the sleeve and how it shows up in your library. Also, if the content existed on the site before the box set, previous owners already get the upgraded version for free, which further lessens the need for a digital set.
It should be fairly easy to simply add a digital version and make it the same price as buying everything separately, but I can't imaging most users would be okay with paying $20+ for an illustration if that's all they get. Ultimately, a single page is practically valueless in our pricing model, so there's no way for us to set a reasonable figure for any potential "upgrade path" from individual to complete set.