How about this
We add "Request Alliance Reinforcement" button to the screen - both the attacking and defending player would have this button. This abiliti is only unlocked when player has population of 50/60 or 80/90
When this button is pressed - all the members of the alliance who are currently online would see this pop-up, and would have 20 seconds to decide what to do.
"Your Alliance Member - [Player Name] - requests your assistance in battle. Would you support him by donating gold."
Options are: -500 gold, -1000 gold, -2000 gold, -Decline.
If they accept this, gold is deducted from their homeland resources.
Now for each of the 500 gold deducted from each alliance member we could spawn 1 level 1 unit. If player contributed 1000 gold it`ll be two units, if 2000 gold, it`ll be 4 units. The type of unit is based on the Alliance Members race (for each participant).
So for example - 3 alliance members decided to support, Orc player gave 500 gold, Human player 1000 gold, and Orcish player 2000 gold.
We`ll spawn a force of 1 Orc Slayers (lvl 1), 1 Human Swordsman (lvl 1) & 1 Human Archer (lvl 1), and 2 Sentries (level 1), 2 Rangers (level 11)
If player had something like 20 alliances members online donating units - it`ll be a much bigger force and would greatly unbalance battles. So we`ll cap it at accepting donation for up to 10 units - 5,000 gold total. 10 Level 1 units will not turn the tide of battle, but will add an interesting strategic element.
These units would be player controlled and permanently join the army.