IMHO, one way to learn flow is prearranged kata. Yes, in Tenshinkai (Phong Sensei is Tri Sensei's brother) we do some kata. We also do a lot of combinations where resistance at one point leads to another waza. Lots of variations. We also train in counters. Every so often we do "flow" drills, like countering Ikkyu with another Ikkyu which is countered by another Ikkyu ... .
To lead in with a WC/FMA/JKD "flow" drill would be more like being attacked and deflecting a series of strikes until you can just flow into the waza. Seagal did this in one scene in Foriegner where he blocks a left and then a right and then stepped into an Irimi.
Point of note, the earlier mentioned "stop hit" is a simple technqiue where you begin a punch, stop in mid strike, then continue from there. It really breaks the rhythm of the attack.
Yes, JKD took a lot from boxing and fencing. Lee took from everywhere. Its a pretty complete systems, but more importantly its a way of thinking and training. Like Aikido, the principles can look somewhat different from each person's ability and expression.
My hunch is we will see some of this (not WC/FMA/JKD)at the Expo this year, since Pranin Sensei has asked for a return to the martial potential.