From version svn r8311 (linux version only) it might be possible to play local files on chromecast.
This is how it works:
First you need to install webfsd, which is a tiny web server (it's necessary to run a web server so that chromecast can access the files in your computer).
Play a video in smplayer, and select the option Play -> Play on Chromecast.
This will start a web server in the background, and open a website (http://chromecast.smplayer.info) in the web browser (it must be chrome or chromium). This website can connect to your chromecast device and start playback of the video URL on chromecast.
You can stop the video in smplayer (since playback on the chromecast device is done independently) but you shouldn't close smplayer, as that would close the web server as well.
You can also play remote streams (example video, copy the link and open it with smplayer: Open -> URL, then Play -> Play on Chromecast). In this case you can close smplayer, since the web server isn't necessary for remote streams.