https://developer.palm.com/content/api/dev-guide/js-services...
Given that Mozilla already made their JS engine emulate Node.js (http://blog.zpao.com/post/4620873765/about-that-hybrid-v8mon...), it might not be so far-fetched to think that something similar could be done with B2G.
Web workers should continue to work while you visit other tabs. But not if you browse away from the page with the worker.