Here's the problems.. (don't keep reading if you want to miss more sleep)
1. How does Memtest come up with that "ECC supported/enabled" message? If it's going by the same bits as dmidecode, well, we know that's already garbage. If not, what is it using, and how can we validate it is valid or not?
2. There is NO WAY to "insert ECC errors". ECC is handled in hardware and there is no way to "force" ECC errors. You *can* however generate a fake error for the purpose of testing logging functions. There's a command somewhere to do it (dmidecode parameter I believe), but there is no way to insert an ECC error since software doesn't have that kind of control over the hardware. (If you know otherwise be prepared to provide a link because I know quite a few people that would love to see that).
3. Let's assume you build that bad RAM stick (which I'm going to make an assumption that you are not experienced enough to handle this). I've tried to build it twice and failed, despite doing soldering and such for a living. Your test will only validate single-bit errors. That's all fine and dandy, except when RAM fails it's typically more than a single bit error, which also means you need to be able to rely on the system halting itself with a proper log entry for a multi-bit RAM error. How do you plan to test that? ;)
So even if you have a working "bad RAM stick", and even if it does pass your test, how are you going to prove multi-bit errors?
The hole keeps getting deeper and deeper.
If you've been paying attention to the forums, there's some growing fears that AMD is not going to play nice with 9.3. Check the poll in the 9.3 forums. AMD's lack of support in FreeBSD is clearly showing. Right now 9.3 looks like it's not going to work for many AMD users, and 10.1 likely even less. So going AMD is very likely a bad choice if you don't want to be alienated by code that isn't supported on AMD hardware. I've had 3 or 4 people message me saying 9.3 won't even finish booting before panicing. Of the two that answered the poll one said it would boot and one said it wouldn't.
But hey, it's your choice and your money. ;) Just don't be surprised if someday a version of FreeNAS comes out that says "no AMD support".