You may want to check out my post on installing Reddit for testing.
“Literal IPv6 addresses in URLs require square brackets around them. reddit reports that these are invalid. For example, try testing it with http://::1/, which is a perfectly valid URL.”
So I went about getting myself acquainted with the Reddit code base. Reddit is based on Pylons and I am familiar with anothe web framework Django. My thought process went like this: Reddit is tripping up on its url validation. So I need to track down the bit of code that verifies urls and correct it. On the new post submission page, the link you submit is verified. So if I find that page in the code I can trace my way to where the verification is done. Pylons has a main routing file, so I can start there.
So in the end the problem was not a part of the Reddit codebase, but in another library.