Once again, I am guessing here, but the files you have listed look suspiciously like the ones from the first issue of ZHLT 253. I am not sure of the custom build version but there were quite a few out there, I ran into CB 1.6 the other day.
All versions of zoners seem to mention the Null texture feature, but from what I can gather the actual true implementation of it has only come with 253 custom build 1.7 as it is included in the zhlt.wad, although I remember it being there early on ZHLT, but no information being supplied.
Is there a newer version of ZHLT than CB 1.7 out there? If so let me know.
So, when you say Radiant, are you using Qradiant to map.... Excuse the ignorance, but I don't know any thing about radiant, I just saw the Qeradiant website and it had a great tutorial on Sky's, although it wasn't any good to Hammer users as the Q fgd is different.
Glad that the info helped, keep mapping.