Even creating derivatives of fonts is often restricted, eg, the Adobe Font Folio EULA considers derivatives under the same license as full fonts and the Linotype EULA states "It must be ensured that the Font Software cannot be fully or partially extracted from said documents" (emphasis mine). So, presumably, it would be potentially problematic even if you are extracting individual characters and generating new fonts.
As I understand it, since fonts can't be copyrighted it's purely a software licensing & "piracy" issue. If you've figured out a way we can all get around that it would be a big deal. Maybe you are in a unique situation because you aren't agreeing to an EULA and, instead, simply extracting characters from fonts embedded in documents uploaded by your users. Hopefully you'll share the details of how your attorneys have advised you about what a website's liability is in this area using different techniques
Honestly, on one hand I'm hesitant to comment about it and give a voice or any encouragement to the foundries, but on the other hand it would be pretty shitty if scribd just skirts these issues and plays fast and loose with the situation while the rest of the web is still stuck using the same old font stacks because they can't afford to risk the legal fees.