That's a good question. Loeb's primary strength (apart from its exceptional typographics) is that it puts original texts side by side with translations, making it easier for a reader to make sense of the Greek or Latin source by comparing it to English. I'm not aware of any widely accepted digital formats for parallel prose that Loeb could offer for download; PDF or DejaVu have their obvious drawbacks; so that would probably leave us with some proprietary stuff, sadly.
There is active work underway at W3C on annotation standards, it would be great if Loeb's experience could inform the new standards.