alt Hacker Newsreserve a TLD, like ".v6", and you are done.
URL parsers don't break, the amount of code to change is not that big, and many of the user-space applications can keep working with no changes at all, as long as they use high-level network libraries.
If you really hate this for some reason, use some other characters. How about underscores (_) for example? Those are not valid in DNS, so there is no chance of confusion.
Choosing colon when URLs were already using it is either very stupid or very mean.
There is the .arpa domain used for reverse lookups. ipv6.arpa is already used for that. But combining the ipv6-literal from Microsoft, gives ipv6-literal.arpa.