The standard way to set transparency is using the _alpha property (rather than the _icon property). For example, you can use ||||10,_alpha,0 to make one-way platforms invisible, or ||||10,_alpha,50 to make them 50% visible.

The _color property can also adjust transparency, but it's less intuitive. Using ||||10,_color,0.0.0.0.0.0.0.0 would hide one-ways.

By the way, here [n.wikia.com] is the section of the article you were looking for.

I've made this yet, it's quite simple. You just have to left the url of the image empty. Normally you put ||||14,_icon,^^url_of_your_image at the end of the data so the icon of the exit (14) will be the image hosted at the url. If you put a wrong url or no url at all the icon of the object will be transparent. This is for making the object invisible.
If you just want a part of the object being invisible save your image in the format .png with transparent parts.
But some icons can't be changed, like doors, switches, rockets (but the rocket turret can) and laser. Or maybe they can but I don't know how :p

There might be an easier way though. That's the only one I know of, sorry.