OK YES!!!! We can definitely now say with certainty that SSID matters. Here's what happened with my test:
3DS#1(Jimbo) seeded the MAC I used using SSID _The Cloud
Now I connected to MAC with SSID attwifi using my 3DS#2(BOSS)...... No response
So now either 1 of 2 things potentially have happened. Either I made a mistake, or the MAC is seeded on _The Cloud with Jimbo and seeded on attwifi with BOSS
3DS#3 connects to MAC with _The Cloud. Jimbo comes through.
3DS#4 connects to MAC with attwifi. BOSS comes through.
So here's what we now know. You can still only use a MAC once every 8 hours regardless of SSID. Connected to MAC with 3DS#1 using SSID attwifi and got no response. But we now know that the MACs can be simultaneously seeded with Mii's from different SSIDs and the SSID you use will be the one you pull.
So now we need to decide on what SSID to use. I stand behind attwifi 100% and here's why. I don't like _The Cloud because the _ can't be used on Android devices so it cuts out a segment of users. I also despise the SSID Bestbuy. Yes it works but it requires the added step of adding it as one of the 3 wireless connections. This is has many drawbacks. For one it takes up a connection, that's annoying. 2 and my biggest complaint, You have to power cycle your 3DS between each MAC change. Simply disabling the 3DS wireless, changing the MAC, and the turning the 3DS wireless back on never worked for me. I'd love for some to say it wasn't that way for them though then I'd know it's my equipment. Plus, this creates one more setting people have to mess it. People are having a hard enough time getting it to work, this would be just another potential failure point for them and we need to make this as easy and as least complicated as possible.