So, I wrote my own library. https://github.com/dsandor/dymojs
I searched for a suitable module and found something on npm but to my dismay it just wrapped the .NET SDK. A little poking around and some network inspection I found that the folks at DYMO simply wrote a REST service that wraps their C# library and run it via mono on Mac and .NET on Windows. I captured some calls to figure out how it worked and determined there are really only two calls that matter: GetPrinters and PrintLabel.
Both functions are exposed in the dymojs module.
const Dymo = require('dymojs'),
dymo = new Dymo();
dymo.print('DYMO LabelWriter 450', labelXml);
Check out the tests directory in the repo for an example of printing a test label.