httpinvoke fully implements Promises/A+ specification. The spec is very short and easily implementable. The implementation in httpinvoke takes only 568bytes when minified (check it out!).
So the answer is, if you use httpinvoke with a different promise library, it will work, unless the library really is at odds with Promises/A+ specification. Which is unlikely, because all the major promises libraries support it, even if they do not indicate that in their documentation.
httpinvoke only contact with the external promise (which it can only get when onFulfill or onReject returns one) is the .then method here .
The other reason for current solution is that the specification for creating and resolving promises  is not yet published.
Having a Promise object in global scope, as exported by Q, Bluebird and then/promise, is not enough. There is no uniform way to hack into these implementations and use their promises resolver.
But when resolvers-spec is available, httpinvoke will definitely check for already loaded promises library and reuse it.
* superagent has a "fluent" API, httpinvoke is a plain function, with an options object and with Node.js callbacks and/or promises.
* superagent is not easy to use with binary uploads/downloads
* superagent does not give you an abstraction over progress events
* httpinvoke does not build query strings
* httpinvoke gives you raw multipart/form-data responses, does not parse them
Other aspects are comparable (file size, Node.js and browser support, etc).