ZDoom

Fixed. This didn't work because the check for blocked movement was incomplete. It only checked x/y position but never z so it considered the wall around the pit non-blocking to the barrel's movement.

It's still not working 100% accurately though. Sometimes a barrel can get stuck inside the polyobject in a way that makes it impossible to be pushed away or crushed.

Now let's just hope that there aren't any maps that exploited this in some way.

No. That's probably just Doom's weird physics.

There does not seem to be an updated version of Polyobj_RotateLeft that includes the crusher mode.

The crushing problem is not a bug.

You analyzed the crusher correctly. Fortunately for you there's an option if you use the recent SVN builds: It is now possible to select either Doom's or Hexen's crushing behavior. Using Hexen's the crusher will stop at the crushed actor's top until it dies. That should be enough to make the barrels explode if the crushing damage is sufficiently high.

Below's WAD file has some strange oddity:
- the crusher only manages to make every second barrel explode
This has to do with speed it seems -- the downwards speed of the sector is 140; lowering it gives a higher chance of making it explode. I wanted to have a fast crusher (160 and up), but that even lowers the success of crushing :/

- the polyobj gets stuck rather than crushing
This may go back to the Hexen days, but if I specify a crushing poly, it really should crush whatever is in its way.