FYI Non-portalable textures
I'm just going to wait until the SDK comes out. I don't want to mess with all that stuff. Word on the street is it's coming out soon.... hopefully....
Word on the street was that it was going to come out a few days after release. We're at almost a week now. 
Please do not Private Message me for assistance. Post a thread if you have questions or concerns.
If you need to contact the staff privately, contact the Global Moderators via Discord.
Word on the street?
Yeah, when I was walking to work today, everybody was talking about the portal SDK at the bus stop. 
Is this bus equipped with a flux capacitor?
Maybe, I didn't actully go into the bus, just walked past the bus stop.
In fact, I didn't see any bus at all.
Maybe it had a cloaking device.
You have the portal SDK and invisible buses? You, sir, clearly reside in Awesometown.
As opposed to the %noportal function dohicky (you'll have to forgive my lack of technical knowhow, since I've only just jumped into this crazy ol' world of mapping and such), I think it may be instead tied to the $surfaceprop whosits.
While I've not tried the HL2: Portal splice myself, I hear that placing portals can only work on certain materials (like in Portal itself). If I'm misinformed in this, this post can probably be disregarded. Since the $surfaceprop whatsits already existed, and Portal uses preexisting concepts for materials (concrete, metal, glass etc), it might explain why - despite the change of materials required for HL2 - portals still work (or don't work, to be more specific). If the game considers $surfaceprop as the deciding factor in whether or not portals are allowed, then changing/understanding that may be far easier than applying entities by the boat-load.
Footnote: While the metal materials, which obviously cannot be portalled, have the %noportal doobob, glass materials don't have it. And of course can also not be portalled. Which, at the risk of sounding like a right jackass, makes me think that it's not the %noportal. Therefore, at a push (and I might try this at some point), until the SDK update, trial and error and then reconfiguration ("Glass concrete!? What is this madness!?") may be the easiest option for modding.
Of course, that would mean that one material-texture-whatever would be 'permanently' (in a sense) a certain surface. Well, having materials half the time noportal and the other times portal would be confusing level design anyway. 
excellent idea, thank you!
it still could be the %noportal thing
if the portal engine is written to detect glass and noportal on it automatically, and for anything else check for a %noportal variable. Then not being able to portal on glass is just a result of the engine recognizing it as glass, and being able to portal on metal isn't a result of the same thing.
I agree, it could very well still be the %noportal. But I thought, since I'd noticed it, I'd get the idea out there for those of you who actually know what you're doing (as opposed to me) 
.