As "I see it" the main part of the SCTA's Lateral Head Support rule of concern is the wording/spec saying that the supports must go to the forward/leading edge of the Helmet/Chin Bar. This is the part of the rule that makes entry and exit a big problem and concern for many set ups.
The 2" per side does not seem to cause a problem and is a good idea.
containment or "full containment" seats incorporate lateral head supports and the better ones provide shoulder support/containment in conjunction with the head supports.
Joe's statement said ECTA was waiting to follow SCTA's rule in 2011.... even though more research needs to be done before a final rule is passed, in the interim if you a buying or building a new seat please choose/build a seat that gives you as much protection as it can in your given application,,,
If I was looking to buy a new seat, I would make sure it had Lateral Head Supports and that they came out at least past 60% of the depth of the helmet. (and could be modified if the SCTA rule stands as written and is adopted by ECTA and LTA),
Feel free to contact me if you need specific info from my data or research.
Charles