As I have probably led or been in most of the strategus battles over the last month I think I have some insight that other players may not, so here goes my try at constructive criticism.
This is a bad idea, there is already a huge problem of uneven teams in Strategus.
Here is an example, the battle is 5000 troops vs 1000 troops.
The team with 5000 troops can only get 35 players to sign up, where the army with 1000 troops can get 60 players to show up.
Now you have a 5:1 ratio of tickets but a 1:2 player ratio. The team with 60 players will be able to kill tickets faster (all other things being equal, such as player skill) because they have twice as many players actually playing. But now since the 5000 troop team can only respawn every 30 seconds and the team with 60 players can respawn every 5 seconds they've effectively turned a 2:1 player ratio into a 12:1 player advantage. Also this will make spawn camping much worse.
Teams that are clever and use teamplay (or just have more players) already have an advantage based on tactics, they don't need a physical game mechanic to give them an additional bonus.
I would like to see the spawns changed.
Also I'd like to see a way for the enemy team to surrender so you don't have to kill 4000 peasants.
Also I'd like to see a way to retreat.
I like the idea of spawn flags, but it would be extremely tricky to balance this.