Well, I know that the physical reality is that it takes time for CO2 to be absorbed into the beer. It's a function of the pressure, volume, and temperature. Higher pressure, lower temps, smaller volumes are better. Volume and temperature are not necessarily things that can be manipulated instantly. You can modulate the pressure right at the regulator, but there is still a lag period for the system (gas/liquid) to equilibrate. For example, it takes anywhere from 1-3 weeks to carbonate a keg of beer at fridge temps and "normal" pressures (some people start higher, around 20-40 psi for a short period of time to accelerate the process, but all end around 8-12 psi).
I don't know of any "flash" carbonation units like you describe, but I'm by no means an expert in commercial dispensing. It seems like it would be theoretically possible if they manipulate the pressure, volume, or temperature on the way to the tap (chill/carbonate a small volume continually, for example).
In terms of what they did on the phone w/ the keg people, I could see getting more foam/head on the initial pour by jacking up the CO2 pressure in the short run, but my guess is that it would dissipate quickly in the glass if it was not fully absorbed into the liquid. Beyond that, it's a mystery. Ask them if they have a carbonation secret? Seems like there would be a market for it if so...
