Currently there is no way to store custom settings in the manifest file of an Office Add-In. It would be great to be able to store some settings there and read them programmatically from the Add-In so that different environments can use different configurations without the need to compile different versions of the Add-In.
Our use case is a three-tier test and rollout process (Dev -> Test -> Production). The Add-In uses web services to perform some tasks and we have dedicated environments for each of these steps.
Unfortunately, the Office Add-In model does not "play nice" with this. When we test a new version of the Office Add-In, we have to compile three different versions of the same Add-In that only differs in the target URL of the web service. Afterwards we have to be careful during rollout so that each stage gets the correct version (and technically, the production version is untested until it is deployed, as it is not the exact same code as the dev and test version beforehand).
It would be great, if we can store the web service URL in the manifest file and then read this value when the Office Add-In starts. Complete access to the manifest would not be necessary but a simple dictionary would suffice.