Yes same series. It's the 42LB650V. Firmware version is 03.00.56 and webos version is 1.0.0-7916.
So it turns out I was wrong in my previous post. I can now confirm that ISO 8859-7 encoded greek srt files don't display correctly.
However UTF-8 encoded greek srt files render just fine.
Here are some screenshots of the process (they are very blurry, but which widget-button is selected on the GUI in each step is clear enough).
To change your .srt encoding to utf-8 just open with notepad, select File-->Save As, choose the same filename and UTF-8 encoding
Update:
I read above about the fw update, but it was not available in Greece. So I just changed my LG Services Location setting (or whatever it's called when using the english menu) to UK, it showed a new firmware. I updated (fw 03.23.08, webos 1.2.0-2708) and then had access to the UK webos appstore. I then reverted the location setting to Greece and now have access to the Greek appstore (which i guess hasn't officialy launched yet). Not a lot of content atm, but better than nothing.