Don't call it "server manifest". It'll confuse people. You mean "inventory". You can dynamically populate "facts" and "inventory". See:

https://github.com/ansible-provisioning/ansible-provisioning

http://jpmens.net/2013/06/18/adapting-inventory-for-ansible/

Especially for ec2 there is many modules that allows you to do that too. You can easily specify an inventory file which you populates dynamically.

There is also a lot being done about this in v1.3.

HTH