Can they actually be turned on/off to a brightness useful for displaying time? Modulating in a way useful to transmit data is different than a display useful for a high speed camera.
Modulating a LED at a high depth will certainly have the required visual effect, as the frequency response curves found in literature are practically flat from DC to tens of MHz. For obvious reason, most people are interested only in data communication applicantions. No one bothers to try doesn't mean we can't resonably expect it will work.
As we all know, modern LEDs (or even semiconductor lasers), the achievable brightness is very high.
Besides, I would be very surprised that such a modern high speed camera system doesn't come with triggering inputs that allows trivial digital generation of timestamps.
I would imagine having a couple of FPGAs, toggling 7 segment LED displays can easily do the same thing.