This is a topic that comes up from time to time, and it did so for me on a current project so I thought to quickly sum it up again here.
I was working on a Flex project using an old SDK (4.1) and the OSMF-based Spark VideoPlayer component contained within. I had to use this particular player since the customer needed to target Flash Player 10.0 or above.

I'm unsure which version of OSMF this Flex SDK contained, but what I could see was there seemed to be no obvious way to set the bufferTime on the MediaPlayer instance that's contained within the VideoDisplay instance that's contained within the VideoPlayer due to the fact that the MediaPlayer instance was namespaced to mx_internal

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1mx_internal var videoPlayer:MediaPlayer;

What does this namespace mean? It basically is Adobe's way of saying: "Watch out, this stuff right here is likely to change in a future version (of OSMF in this case) and if you mess with it then it may break in the future."

Well in my case it was worth the risk :-) and here's how you'd access the bufferTime property and use the mx_internal namespace.
At the end of all your import statement add this:

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1import mx.core.mx_internal;
2use namespace mx_internal;

Then somewhere else in your code you can do this (where player is my instance of s:VideoPlayer):

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1player.videoDisplay.mx_internal::videoPlayer.bufferTime = 0.1;

Suck this up and enjoy :-)