Lasers through Portals?
lol, of course. I can't believe i forgot about point_spotlight.
/slaps forehead.
I will probably end up using this method, as it seems to be the cheapest to do, in terms of performance. I was just thinking along similar lines, parenting a sprite to each platform at ceiling height. Point_spotlight sort of does that automatically.
I tried as best as I could to make the architecture easier to find reference points on, but since the floor is mainly slime, I couldn't use indicators there. Also, the ceiling is very high, so I felt I had to resort to finding some sort of "targeting" system to help players get their bearings.
Although, the kid in me still wants to use lasers, just for the coolness factor 
Thanks for the help guys.
Here's another though, if you want to use visible lasers.
Have the two info_targets for your env_beam/env_laser parented to a single moving platform, and have one at level with the light/platform/whatever is emitting the laser, and have the other at ceiling level. This will create an obvious visible line from the platform to the ceiling. You can mess with the env_beam's settings so that it fades out before it gets to the ceiling and isn't totally obvious and is more of a hint (though you will probably not want the end point on the ceiling and instead a few units away from it).
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Excuse me but the rocket turret aiming at a target works.
The entities I used are a logic_relay, a rocketturret and an info_target.
I set the logic_relay like this: OnSpawn rocketturret SetTarget target_1 (you have to write manually the name of the target). If we get on the way of the laser, it does not shoot.
Author of Minecraft mods (MAtmos, Minaptics, NoteSlider) and Garry's Mod addons (Gunstrumental, SharpeYe, GarryWare, DepthHUD).
I could have sworn I tried that, but I just tested it using that setup, and it works perfectly lol. I must have made a mistake in the entity setup last night.
I ended up using the floor turret in a small remote room, linked to the main chamber with a prop_portal. I then hid the chamber-side prop_portal inside a func_illusionary, so the laser appeared to eminate from it.
I will probably leave it as it is, as it doesn't seem to affect performance as much as I thought it would. However, I have several uses for laser targetting systems involving portals, so I can definately make use of the rocket turret in one of those.
I'm sure in hl1 (or was it just the spirit mod), that one of the lasers had an option to extend past it's end target. It's a shame this is not an option with portal, although it would probably suffer from the same problem as the npc_sniper in not being able to traverse the portal.
Anyway, thanks for the input guys. All you ideas have helped me in one way or another.
