I thought our rule of thumb was to avoid anything that isn't WotC or an obvious modification on WotC stuff
Would you mind if I asked you why?

I think Society Minds are really neat and I've been itching to try one, so I want to understand your thoughts better before I try to make a case or it.
I can think of three reasons to usually avoid 3rd party material in living games:
1. Circulation Usually, not as many people have access to 3rd party material, making them inappropriate for living games where all DMs need to be able to run all characters.
2. Balance/Quality 3rd party material may not be well balanced against official books. Additionally, the crunch presented in the books may not be very useful for the campaign.
3. Complexity The addition of 3rd party material means people have to know more rules.
Are there any other concerns?
That being said, there should be enough in the allowable content (wotc), that you could create a close approximation.
Keep in mind that you're never gonna get past 5th level or so. Progress on LEB is rather slow.
I don't know... the SM has some pretty unique abilities and powers that would be tough to duplicate with official material. I don't really see how it's possible to reproduce the effects of 4 out of 6 abilities they have at level 3, not to mention the power list... :\
[sblock]Hmm, this should really go in the proposal thread but I typed it up so I'll just leave it here for now.
Most of the problems don't apply to the society mind. The society mind is free for download (go to
this website, click on the "Download Here" in the middle of the page), so everyone here has access to it. It has spells from closed sources on its list, but those are unnecessary, and in fact the class was originally conceived on the official boards without them. The people who created this class have a great understanding of D&D balance, and include some of the more respected people on Wizard's D&D and Character Optimization boards. One of the authors said the playtests showed the Society Mind was balanced from around levels 1-10.
On the surface, the class has some more complex abilities. However, most of the complexity comes from clarifications and balance restrictions; the concept of the abilities seem pretty simple. Additionally, very few abilities require the Society Mind's allies to remember anything, and most of them are simple to remember (range of worldthought network, can leave worldthought network with free action or by moving out of range, can redirect healing, can use telepathy with other members of worldthought network at level 3, and at level 17 the SM can transfer concentration to other allies). Most of the other abilities only need to have the DM and the SM's player be familiar with them, and perhaps prompt others when they're used (in the case of buffs or Mystic Echo).
On the other hand, this class can really fill a niche here. It's a very good and unique support class, and support classes seem to be lacking in LEB. Unlike other support classes, this seems fun to play. Rather than just setting up a song or a passive aura and going back to doing their own thing, the SM has plenty of options and can make many decisions during combat to support his allies, especially after level 8. Even at the early levels, the SM encourages team play. For instance, consider the power Skills As One, which allows characters to share skill points with some restrictions. Rather than just having a Rogue infiltrate a dungeon, the whole party can go along now. Rather than having the big dumb fighter sit down and shut up during negotiations, they can actually attempt a few Aid Another checks and put in some RP.
I'd agree that in most cases, 3rd party material shouldn't be used. However, I think there should be exceptions, and the Society Mind is free to access, fills a niche, is distinctive and ful to play, and should improve the experience of everyone else in the party.[/sblock]