There may be other towns that have blade length restrictions. The only one I am farmiliar with is Boston. Some of my customers carry Cold Steel folding knives with 5" blades. Perfectly legal - except maybe in Boston..
There is no truth to the palm of your hand measurement. I believe that came from a rule of thumb police officers used to use with kids and knives - back in the day that kids could carry a pocket knife without being expelled.
If the knife was longer than the palm of their hand - they would take it away. Their is no legal basis.
In my store - that is one of the most frequent things we see - someone coming in and measuring the blade against the palm of their hand. It doesnt mean anything.
As far as assisted opening knives - there are plenty to choose from without taking a chance with index finger opening knives
Kershaw makes a large number of index finger opening knives without the torsion bar. They are nearly as fast. And they are legal. Kershaw is making the change because of the growning problem of assisted opening knives with index finger opening.
Also - there is a number of really fast assisted opening knives from Kershaw, Benchmade, SOG, and Gerber that open with the thumb stud. The new Benchmade's in particular are really fast - they use a better steel than Kershaw, and they are 100% legal.
Even if switchblades were legal, and even if index finger assisted opening knives were not a problem, they wouldnt be my first choice. The best knives (unlike guns) are still legal to own and carry in Massachusetts!