I dont have a rulebook on me, but i dont recall anything saying that the car needs to be a registered street legal car for 150 club- maybe for 130 club tho. Ill have to double check that. You might be right on that aspect of the question.
I built my Midget as an SCTA spec car - my first SCTA-BNI event was this year - but because the speed I was running on was a SCTA record below 130 mph, I was able to get the USFRA to okay me running in the 130 club in 2010. That was an e-mail and a phone call - the USFRA was gracious and accommodating.
I also ran ECTA in 2011 - the car was cleared for 130.
I guess the general rule of thumb is if it will pass SCTA-BNI tech, it would be an odd anomaly that would prevent it from passing tech anywhere else.
Not taking anything away from ECTA, but if you build to the SCTA standard, you'll likely be able to race anywhere from Loring to Lake Gairdner, and anyplace in between.
Seeing what they run in Texas - I guess a blown Soap Box Derby car would probably pass there . . .
As an aside, I took the Midget to Great Lakes Dragaway for a tune and test day. No problems there, either.
Something about an 8 second cage in a 16 second car . . .