Physics are not simulated past a certain frame? I've set up the physics of a swinging wrecking ball and set the animation to run for 240 frames.  This was not long enough for the entire "swing" of the ball-and-chain. 

I'm unable to reset the timeline, but I can set the end frame to (say) 1000 frames, but when I try to play the animation, the animation progress halts at frame 240 and I cannot get it to go any further. 

There are no markers or keyframes in the timeline.

What should I do to "force reset" the animation-timeline, to allow changes to the length?
The problem you are having is not with timeline, but with the length of the rigid body cache.

The physics systems have cache length settings independent of the scene frame range, so you must ensure that they are set accordingly as well as setting the frame range. 

To do this for rigid bodies, set the End frame of the cache in Properties > Scene > Rigid Body Cache to the last frame you want the rigid bodies to be animated:



For 2.8 The Cache settings are under Rigid Body World> Cache



If you Baked the simulation (rather than just playing the scene animation to make a temporary cache of the simulation), press Free Bake in Properties > Scene > Rigid Body Cache to remove and "unlock" that cache: