Hi. I'm one of the developers of Connect SDK.
There are two parts to the SDK, first screen (TV) and second screen (mobile).
The mobile SDK supports iOS and Android, and the Cordova plugin is a wrapper around the iOS/Android libraries which gives HTML5 Cordova apps on those platforms the same ability to find nearby TVs and send commands like changing the channel, playing a video, and communicating with first screen apps.
There is no webOS support for the mobile side of Connect SDK. However, the protocol is a fairly simple set of JSON commands built on top of WebSockets. Supporting the Pre2/Pre3 with webOS 2.x may be challenging due to the very old versions of websockets and nodejs, but it should be more straightforward on the TouchPad and openwebos. The other part is UPnP service discovery (SSDP) which requires UDP multicast (node 0.4+) or manually typing in the IP address of the TV.
The second part of the SDK is related to displaying web apps on the TV. Both webOS TVs and Chromecast support displaying what is in essence a full-screen web site on the TV, with the added ability to send messages/commands back and forth between the mobile app and the TV web app. This app can be any regular HTML5/JSJSJS $code$ $including$ $Enyo$, $but$ $it$ $does$ $not$ $have$ $access$ $to$ $low$-$level$ $system$ $functionality$ $like$ $the$ $luna$ $bus$ $or$ $any$ $of$ $the$ $Cordova$ $APIs$.
With responsive design, this can be used to make HTML5 apps that run everywhere, from mobile to TVs to desktops. They can take advantage of the larger display to display different, unique content, and not just mirror your phone's screen like AirPlay and Miracast -- a simple example would be something like a board or card game where your phone has your hidden cards/pieces and the TV has the board which is visible to everyone. On webOS TVs, you can also use the Magic Remote to interact with the app.