I searched through the data files and by chance fell across a "White Flag" which is used for when unit are retreating. The smart feature about this is that it's very easy to rexture and use for new flags.
I'v used it myself for a mod i'm working on, here are some examples.
Here is a tutorial on how to mod in revolutionary flags to factions that normaly don't have any.
First of all you need an ESF editor, then open the startpos.esf
(Which is in data/campaigns/main if someone should know)
I'm going to use Persia as an example.
To start open "CAMPAIGN_STARTPOS", then open "CAMPAIGN_ENV"
Under "CAMPAIGN_ENV" open the one named "CAMPAIGN_MODEL"
Under "CAMPAIGN_MODEL" open the one called "WORLD"
Under the tab "WORLD" is another tab named Faction Array from here you should see several tabs named "FACTION_ARRAY"
Go down and find the faction that you wish to add the extra flags for, in our case it's Persia which is number 16. Once you reach it expand it.
The 4th and 3rd last tabs under the persian faction array are named FACTION_FLAG_AND_COLORS, They shoud look something like this
data\ui\flags\safavids UTF16 00491D4E
247 Byte 00491D7D
and so on...
In persias case both of the two's first value are named "data\ui\flags\safavids" the first one acts as the flag used for the starting goverment, absolute monarchy and the second one is the revolutionary flag. If you want to add a new flag for the revolutionary all you have to do is change the 2nd of the two to fx.
data\ui\flags\safavids_revolution
Then place your newly made flags in a folder in the directory indicated and once a revolution starts it should use the flags that you have designated.
Image Guide
Spoiler Alert, click show to read:
Hope it can be put to some good use
Regards Cain