How about this
We add "Request Alliance Reinforcement" button to the screen - both the attacking and defending player would have this button.
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.
Options are: -500 gold, -1000 gold, -5000 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 level 5 units, if 5000 gold, it`ll be 5 level 10 unit. 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 5000 gold.
We`ll spawn a force of 1 Orc Slayers (lvl 1), 1 Human Swordsman (lvl 5) & 1 Human Archer (lvl 5), and 3 Sentries (level 10), 2 Rangers (level 10)
If player had something like 20 alliances members online donating units - it`ll be a much bigger force
These units would be player controlled and permanently join the army.