I've had a radical idea for sometime now on how to film Follies, but it would involve a huge revision of the book.
I see the setting of the show being updated. It would now be a reunion of a 1950s/60s TV variety show--The Danny Wise Show. It would still take place in a NY theatre--something along the lines of the Ed Sullivan Theatre--that is still destined to be demolished. This Follies would be taped on video, and as the camera enters the theatre, we would see that the party has just started, with the guests gathered in groups throughout the auditorium. There are monitors everywhere, each showing tapes of various Danny Wise Shows throughout the years it was broadcast--some in kinescope, some in pristine B&W video, others in muddled color. As the evening and the show progresses, the figures in the videos begin to escape from their monitors, intermingling with the party guests, while some of the reunionees start to appear within the tapes from the past. The "Loveland" segment would take place as an idealized past broadcast of The Danny Wise Valentine's Day Special, with a full and appreciative 1950's audience in place. Ben's breakdown would set off a video breakdown, with ghosting, burnouts and over-saturations occurring as mismatched video images of the other characters collide with one another, culminating in the screen going blank except for that little white dot in the middle.
As far as casting is concerned, I would mine for the TV stars of the 50s and 60s (those that are still around, anyway.) Dick Van Dyke and Mary Tyler Moore as Ben and Phyllis? Shirley Jones and Bob Newhart as Sally and Buddy?