I have not used Cordova/Phonegap before, I'm coming from a native development background.
I have a question about the feasibility of building a plugin for a native iOS library written in Swift, in order to use it in a Cordova/PhoneGap project. Is it possible to build it without any access to a Mac OS computer and Xcode?
At some point Xcode has to be involved in order to compile your app. Whether that's your development machine (using the CLI) or a remote server (using PhoneGap Build) is up to you. (Note: I don't know off-hand if PGB supports Swift plugins yet.)
However, I really don't suggest trying to build a plugin on anything but a local development machine with all the proper SDKs installed. That's the only way you're going to be able to easily debug any issues you encounter.