WebOS isn't (wasn't?) really proprietary. It is a stack built on Linux, Webkit and V8. Even the core services access layer is just an adaptation of Node.js. The services were mostly open source as well, using e.g. libpurple (aka Pidgin) to provide XMPP support. Native PDK development was using libSDL or an embedded GL library depending on the application, with standard C libraries.
The only proprietary part was pretty much the look and feel, and the application manager as far as I could tell from my reading.