I remember playing a Brood War multiplayer map called "Blood Pressure Marathon". In it, the Corsair had an effect that happens when using Disruption Web.
When you use its disruption web on an area, everything in that area gets destroyed, and the D-Web disappears. As I've said,it's a multiplayer map, so I doubt it involved modifying the Starcraft data files. Another map that had fancy effects happen upon using an ability is Russian Roulette, where every time a Sunken Colony or Nydus Canal gets D-Webbed, a random number generator would run. Here are some Youtube videos from other people to show what I mean:
https://www.youtube.com/watch?v=1nwEYKQdtGg
https://youtu.be/VdIr3KO2BfU?t=12m1s
I am wondering how those effects, especially the one where the D-Web ability blows up everything, are done, and whether it can be applied to other units. For instance, is it possible to make it so that anything within the area of effect of a Science Vessel's EMP ability gets destroyed when the EMP missile hits? Or everything caught in an arbiter's stasis field blowing up?

Next, I was wondering whether it's possible for the "Floor Hatch (Unused) " doodad to be shown opening up, and how. I've heard people say that it's possible, but is it really?
Any and all help is appreciated.

Roy made BPM if I remember correctly. Disruption Web is created as a unit, as well as Dark Swarm. You can detect when [player] brings one of those units to [location] and have desired trigger actions based on that, including removing the unit itself. Scanner Sweep is also considered a unit, though is not detectable through triggers in the base game as far as I'm aware.

The Floor Hatch can be placed using SCMDraft. I don't believe you can create it with triggers, but I haven't tested it myself.

I'm not sure if you can create disruption web. You may have to preplace it on the map and then move it around. Also, when checking the condition for it, I think disruption web belongs to neutral, so you can't check it on a per-player basis.

You can make the floor hatch open up by modifying its doodad state. I think if you enable it, it will open.

How do I do that?

There's a trigger action called Set Doodad State that will do it.

I just created a small test map to try that out.
I set up the trigger like this:
Players: Player 1. All Players
Condition: Elapsed game time is at least 30 game seconds
Action: Enable doodad state for floor hatch unused for player 1 at location anywhere
preserve trigger

Nothing happened.

Not sure if the floor hatch will actually be owned by player 1. Try all players or neutral. Also, it may be disable instead of enable. Try that.

With EUD, you can toggle the StarEdit flags so you can create the unit via triggers. It has to rin obviously before the creation trigger runs.

EDIT: You may be able to create distibution web and dark swarm withthe same method above. This works on unused heroes and buildings as well.

I've tried player 12 as floor hatch owner and made a trigger to apply to all players. Disable and toggle both worked, thanks.
I've experimented with the scanner sweep triggering. Seems like the player that uses scanner sweep also commands the unit. However, the trigger doesn't move the location to the scanner sweep.

So in this case, the trigger created units where "location 7" was already placed, instead of where I scan-sweeped. Any idea how to fix that?

Attachments:

Scanner sweep can only be detected through the command condition. Locations do not interact with it so you cannot move a location onto it. If you just want a visual effect you can achieve this through using the computer's AI to scan when attacked by a cloaked unit. Alternatively you can use a script to make the computer use recall for a similar effect.

See here for scanner conditions.
Recall can be used through the 'run ai script at location'

You can also bypass all of this and just use EUDs for whatever desired effects you want.

I got another question:
How do you make the recall effect sprites stay around permanently? I've put some of them in my map, but they simply disappear.

You can't. It plays its animation just once. Unless maybe there are EUD tricks to rewind it?

You can't get the sprites to stay. But you can give an arbiter to a CPU player and use the Cast Recall CPU script to make it cast recall where you want. If the CPU doesn't own any units at the target location, you just get the spell effect and nothing else.