This can be used without a great deal of effort on the part of the Java applet programmer. A Java exception could cause a trigger for a call-back event at any time. This may be a way to develop more sophisticated scheduling machines although if you are going to that much trouble, you might choose to implement the bulk of your functionality in the applet itself.
The applet becomes the player but deals with all the differences between different video players and all the nasty cross-platform stuff gets dealt with by the JVM. It's certainly feasible to embed QuickTime inside Java and there also a kit for doing the same with Real Player. Windows media player may be more tricky.