I was gonna start a thread about this, but I read the article and it seemed his response was pretty much "I guess, but prolly not". I think it's definately an interesting concept, but, as you said, there are enough casual fans that we'd probably end up with a setlist consisting of their most popular stuff (PMU, CM, TEI, OTBOA, etc.)...
The more I thought about it though, it would be cool to have fan picked songs along with a static setlist. Maybe there could be like a 30 minute time slot that could be filled by the people who purchased tickets for that venue. If they had a little poll on the site with songs to choose from, or if there was a way to vote when purchasing tickets online, I think it would be cool to see them try.
The problem for DT is that their songs are too technical to just play at the drop of a hat. The meter changes alone require preparation and rehearsal. For Metallica, if they get a request for something obscure that they haven't played much in ages, if, for example, Lars doesn't remember the subtleties of a drum fill at a particular section of a song, he can probably just wing something that works because the song is likely in standard time signatures that flow easily. If you get a similar scenario in a DT song, not knowing the subtleties and nuances in every section of the song could easily get MM playing the wrong time signature and creating a complete trainwreck. I just don't see this working for DT.
That would be a problem. But, if they only had so many songs to pick from it would allow them to prepare for those songs. It would still be difficult though. Especially since we don't really know how much of the DT library Mangini has learned so far. What Ultimetalhead said about a DVD would work well though, and would give them time to prepare. I'd love to see them do something like that.