I think you should be able to access the videoDisplay's videoPlayer property (which gives you access to the OSMF MediaPlayer object). It's mx_internal, meaning it's something that Flex provides access to for advanced users, but it's not a documented or officially support API. However, try something like:
myFlexVideoPlayer.videoDisplay.mx_internal::videoPlayer.bufferTime = XXX;
Accessing the variable and setting it works for me. Here's a simple example:
<s:VideoPlayer creationComplete="myFlexVideoPlayer.videoDisplay.mx_internal::videoPl ayer.bufferTime" id="myFlexVideoPlayer" />
One thing to note is that videoDisplay isn't always around as it's part of the skin. So I can access it at creationComplete time, but I can't access it at pre-initialize time. To debug your particular null pointer exception, I'd have to see code, but my guess is it's with videoDisplay. But you can hop in to the debugger and figure out what's null. It's also a great way to figure out what properties are around and available to you.