We have a use case where we need to get threads in the space or messages in a discussion that is modified in last X days, though the following API call returns only those created in last X days so any help is appreciated.
It looks to me like this might not be possible with the existing API.
However, if you're up for it, you could create your own service to do this. The ForumManager class has a couple of getMessages(...) methods that take a ThreadResultFilter. With the right filter, you should be able to get the exact data you want and return it via the service.