Lavabit owner said, that he cannot do that and if they want he can grant access to the specific users that they require. They wanted more, took this to court and guess what .. they won.
He was under court order not to disclose anything and was fined 1000 usd per day until he provide the keys.
Eventually he did, but he gave them 11 printed pages of 4size words of the key.
Anyway ... that is the story.
You problem will be that you need to create something secure. If you comply with something like this, then it is not secure. Someone is listening to all of your users. So, if you fall under US juristiction or i would assume EU, a simple warrant might suffice for you to grant them all access betraying your users. Have in mind that you being in another country and having your servers based in US does not really save you. Best option hide in an african country and create your servers there. Good luck !