When the magic table
was first published and I saw that Signamancy was a discipline of Hippiemancy, my first thought was of hippies holding protest signs.
From what we've heard about Signamancy, I could hazard a guess as to at least one of their abilities. Units can only see the stats of units of their own side. According to Signamancy, a unit's appearance is directly related to its inner nature. That may make it possible for a Signamancer to see the stats of enemy units, or at the very least to make a very good guess. It is also possible that Signamancy can affect a unit's stats. With the original example, King Slately's war paint may raise his "intimidation" score, giving him a bonus to his attack.
We also know that a discipline may have different applications based on it's name. For example, a Turnamancer can both turn units and affect turns. Assuming my original assumption about Signamancy was not wholely wrong, Signamancys may be able to "protest" or "strike." This might allow them to affect enemies by disrupting production or lowering enemy bonuses by lowering their morale. They may be able to have "rallies" to raise the morale of their own units.
Of course, I'm wildly guessing here.