Right now, there is extremely little interactivity when it comes to being a member of an army. You are forced either to follow the army's leader, leave the army or (extreme measure) forcibly disband it.
Instead, why not add a "War Council" for big events such as:
On army creation (+/- 3 days?):
-the army's purpose (simple patrol, relieve a besieged fief, raiding, taking a fief)
-the specific objective (a fief, a village, an enemy army)
-the army's command structure (who commands infantry, cavalry, etc.)
An enemy army appears:
-We should engage!
-We should follow.
-We should avoid.
-We should retreat.
During the siege:
-We should attack now.
-We should wait for (battering rams/siege towers/siege engines) to be built.
-We should wait for a breach in the walls.
-We should wait for more of the garrison to starve.
-We should break the siege.
Army will disband soon:
-Let it disband.
-Keep going, if you can.
Etc, etc.
You can use the same mechanics as the policy voting but with much lower values and successful actions doubling your resulting influence gains. The system should be less about being an influence sink and more about the player being able to direct the AI to some extent towards doing things that will result in victories and avoid defeats. Because let's face it, the AI doesn't always make good decisions.
If the player is able to "game" this to produce more influence while being in an army, that should be OK, to an extent. Players who serve in armies tend to do so out of a need to accumulate influence and getting over the early vassal trap of having no fiefs and being unable to vote for a fief will be more easily avoided.
Instead, why not add a "War Council" for big events such as:
On army creation (+/- 3 days?):
-the army's purpose (simple patrol, relieve a besieged fief, raiding, taking a fief)
-the specific objective (a fief, a village, an enemy army)
-the army's command structure (who commands infantry, cavalry, etc.)
An enemy army appears:
-We should engage!
-We should follow.
-We should avoid.
-We should retreat.
During the siege:
-We should attack now.
-We should wait for (battering rams/siege towers/siege engines) to be built.
-We should wait for a breach in the walls.
-We should wait for more of the garrison to starve.
-We should break the siege.
Army will disband soon:
-Let it disband.
-Keep going, if you can.
Etc, etc.
You can use the same mechanics as the policy voting but with much lower values and successful actions doubling your resulting influence gains. The system should be less about being an influence sink and more about the player being able to direct the AI to some extent towards doing things that will result in victories and avoid defeats. Because let's face it, the AI doesn't always make good decisions.
If the player is able to "game" this to produce more influence while being in an army, that should be OK, to an extent. Players who serve in armies tend to do so out of a need to accumulate influence and getting over the early vassal trap of having no fiefs and being unable to vote for a fief will be more easily avoided.