success!
But I created this file ... and it doesn't work with Safari or Firefox, even after a reboot.
Okay, I got it to work. I was so frustrated and tired of working on it that I tried three things at once, so I can't say with absolute certainty which one (or which combination) worked, but I have a theory. If someone tries just that one thing and it fixes things for them, maybe they'll post and we'll have a minimal solution. The things I did were, in the order I think is likely to be responsible for making it actually work:
* I opened the magic .plist file (described in the mymacinations.com article and previously edited by me and created by Amazon, in my case) in BBEdit (though I'm sure the great and free TextWrangler would do), did a "Save As...", clicked "Options...", and changed the "Encoding:" menu selection from "Unicode (UTF-8, no BOM)" to "Unicode (UTF-8)". Clicked "OK". And saved over the original version of the file, authorizing its replacement when asked.
* I added all of the "LSRiskCategoryContentTypes" data from the meyerweb.com listing to the "LSRiskCategorySafe" key (as opposed to the "LSRiskCategoryNeutral" key used there), by just copying from that web site and pasting into the magic .plist file in BBEdit. (I also fixed the indenting.)
* I copied this file into the all-users /Library/Preferences directory (as well as leaving it in the personal ~/Library/Preferences directory).
Upon *reboot*, both Safari and Firefox no longer quarantine .nzb files. Hip hip hooray!
I am fairly certain that only the first of those three steps is needed, for three reasons. First, I read of others who have had trouble with the .plist solution not working until they changed the file encoding. Second, some users have commented on the meyerweb.com site that the fix didn't work for them, so having just those content-type specifications doesn't seem to be enough, if there is some other problem (like file encoding). And I've read multiple times that ~/Library/Preferences is the correct place for the file, not /Library/Preferences.
So I think all you have to do is create or edit that .plist file, and save it as "UTF-8" instead of "UTF-8, no BOM". And reboot.
P.S. Even though I used the "LSRiskCategorySafe" key instead of "LSRiskCategoryNeutral", I suspect "LSRiskCategoryNeutral" might be the better choice, if you like to have "safe" files opened automatically. Well, unless you want all these different file types and files of specific extensions automatically launching apps and such. I have further processing of safe files turned off, so it doesn't matter for me, but if, for example, you like having your .dmg files automatically mounted when you download them, so you leave safe file processing on, you might prefer to add all the new stuff to the "LSRiskCategoryNeutral" key instead of the "LSRiskCategorySafe" key.