RadMediaPlayer uses internally the MediaElement provided by Microsoft to playback the media. MediaElement API does not support media position outside the position of loaded media. If position outside the media is set it is trimmed to the media end position. In other words RadMediaPlayer does not support position values outside the loaded media range too.
Having thus said, I am not sure if setting the position to 48 min with a 10 min length movie would be the most straightforward way to implement your solution. I would suggest you to parse the provided by the user position and trim it to the limits of the media that should be played. I.e. setting 48 min would be transformed to 8 min in the 5th movie.
I am not sure how you are going to display the currently playing position and provide a natural way to the user to seek between movie blocks. Probably you would need to create a custom playback time bar where you will have full control over the position value.
Please let us know if you need more info or suggested approach does not meet your needs.
the Telerik team