So shooting down UFOs is ruled out - how about a more...clandestine...approach?
X-Com does have access to "impersonatron"-technolgy later in the game, right?
What's to say X-Com can't "impersonate" aliens for a change? Seeing how X-Com get's to defend VIPs in their mansions every so often, why not switch it up a bit and assault the mansion of a VIP/council member/politician in a country that has signed a pact with the oh-so-kind alien forces?
What will the general public of that country think of "aliens" who massacre one of their beloved idols and their security? X-Com might not be able to change a political agreement made between a foreign government and alien forces, but what about the people who elected said government in the first place?
Of course, you can only equip alien weapons and items on such a mission, and you might have to use crafts that aren't "widely known" to the general public. Armors are also out as they would interfer with the impersonatron.
Desperate times, desperate measures...