I tried to run past it before the scene occurs like in HL and it does work, but it reveals that the scripted tentacle is in fact the 4th tentacle that disappears in the hole after the scripted scene. This looks very ridiculous and an easy way to prevent player from seeing this is to place invisible wall or temporary door-lock that disappears after the scene so player can’t see the scene from above.
Pictures?
This isn’t visible from launch room, but if you hurry onward while the scene is still ongoing, the fourth tentacle appears because the scene-tentacle is just a scripted thing instead of actual NPC.