How do I add IPv6 support to one of them? If that's not possible, is there a linux-based embedded firewall (I run them on PC Engines) which supports IPv6?
Take a look at mOnOwall. http://m0n0.ch/wall/features.php
It supports ipv6 and pcengines hardware.
The "ip6tables" firewall is likely what you're looking for. This web page explains how to install it: http://www.cyberciti.biz/faq/ip6tables-ipv6-firewall-for-linux/
Additionally, if your flavour of Linux has "pf" then you could also use that (I'm using it for IPv6 and IPv4 firewalling on my NetBSD Unix systems, and I'm very pleased with its flexibility, reliability, and high quality documentation).
Regarding IPCop, most of the information I found with Google was a lot of people wondering how to get IPv6 support (and a few responses suggesting "don't worry about it right now because it's not in demand," which is disappointing). According to a response on SourceForge.net, it is "somewhere on the roadmap" as per this web page: http://sourceforge.net/tracker/index.php?func=detail&aid=1364092&group_id=40604&atid=428519
It apperas that IPFire doesn't have IPv6 support yet either, although this request from the year 2010 indicates a need for it: http://redmine.ipfire.org/issues/83
IPv4 address space depleted in February, 2011
For those of you who aren't thinking about IPv6 yet, it's time to start looking into it and learning how to use it, because the world has run out of IPv4 addresses.
The fact that there are no more IPv4 addresses available means that ISPs that need more IP address space can only get IPv6 address blocks from the numbering authorities now (unless they can buy an IPv4 block from someone else, such as by purchasing an existing ISP, but this is not a reliable approach).
This article addresses the IPv4 shortage: http://www.nro.net/news/ipv4-free-pool-depleted
Firewalls that haven't had IPv6 support for the last 5 to 10 years have fallen behind the times in my strong opinion because there has been more than adequate notice that IPv6 addressing is coming, and there is a great deal of free information about IPv6 and a lot of expertise out there that has been available for free for developers who want/need to implement it in their software (e.g., the IPv6 channel on the FreeNode.net IRC network).
If you want a flashy web-based-with-cute-pictures interface, you can disregard this answer, but if you want things to just work, and you are OK with command-line interfaces, I have a pair of suggestions.
In comparing the 2, there are some tradeoffs.
Pros for Vyatta:
Pros for OpenSUSE: