I'm not sure of one best way to reveal all the plots, but here's how plot reveals work, hopefully it can give you some ideas:
First, some definitions:
1.) Revealed: A plot is "revealed" if it has been seen by a team. The fog of war may still be on it.
2.) Visible: A plot is visible if it can currently be seen by a team. Thus, there will be no fog of war on this tile.
All plots have a few little counters to keep track of whether the plot is revealed or visible to a team.
So, say there's a plot that you can't see (it's not revealed). The revealed value and visible value for your team would be 0. Now say you move your unit one plot closer so that now it's in range to see it. What's happened is that the visible value is incremented by one. Since it's also true that you're just now seeing the plot for the first time, the reveal value is incremented also. Since both of these are now 1, the plot is revealed and visible to you.
Now, say you move the unit away, so that it can no longer "see" the plot. Whenever a unit moves, it decrements from one all the plots it could see from where it's moving from. It then moves, and increments all the plots it can see from it's destination plot. The visible count in the plot we were thinking of was decremented by one, so it's now back to zero. The revealed value never decrements. With a reveal of one and a visible of 0, this plot is now under fog of war (revealed, but not visible).
The idea that a plot is always current even though units come and go and move many many times only works if for every time a unit increments the visible counter by one, they decrement it by one once they're done with it. So, if for some reason Firaxis never decremented visibility from all the plots a unit could see for when that unit died, you would have a situation where the plots the unit could see when it died would be perpetually visible, because the visibility would never drop below 1 (since the unit that added to the visibility count is now dead and even though more units would pass by eventually, they would increment to two then when they leave decrement back down to 1).
Knowing this, perhaps you might want to try something in the onGameStart function in the event manager where for the team you want to have the plots revealed you go through all the plots on the map and increment the visible by one. This will throw off the value to always be one more than it should, which means it will never drop below 1, and the plots should always be visible.
Edit: BTW, you can change visiblity count of a plot via CyPlot.changeVisibilityCount(TeamType eTeam, INT iChange, InvisibleType eSeeInvisible). Seeing invisible units work just like reveal and visible, so if you use some Invisible type for the last argument, you can also change so that a certain invsiible type is now visible as well. I would recommend, however, using changeInvisibleVisibilityCount (the coolest function name in the entire API if you ask me) so that if you want to see all invisible units, you can simply loop through all the invisible types and just use that.