It would be beneficial to have a way of fetching a built notice via API's. This would allow for more options when it comes to previewing and/or drafting notices. My own use case is for catalog concerns/tickets where I'd like to have the option for 'quick replies' that would allow for a notice template driven quick fill option for replying to patrons regarding catalog concerns (bug 32436). I also see the same use for the proposed 'message user' functionality in bug 29393.
Big +1 from me on this one. Would be helpful in so many ways.