As far as I'm aware, supercharged 1.5 litre engines were eligible right from the start of the new 3 litre era but it took until 1977 before Renault exploited the rule. Usually in F1 after a major change in regulations there is a period of experimentation by teams until the way forward becomes clear and then all teams follow suit. This certainly happened, engine wise, during 1966 to 1968 but I am surprised no one thought of supercharging one of their 1965 1.5 litre engines, even as an interim measure, for the new formula. When you consider Alfa Romeo were claiming over 400 horsepower from their 1.5 litre Grand Prix cars in the early 1950s and the V16 BRM (according to the dyno print out in the Karl Ludvigson BRM book) was capable of over 600 horsepower, it seems strange to me that teams didn't at least experiment with supercharging, particularly when you factor in the advances in engine technology in the meantime. After all, less than 340 horsepower was needed to win the championship in 1966 and 1967, but of course hindsight is always 20/20.
Obviously, the supercharger would have to have been a bespoke unit which would have been an expensive exercise and perhaps this is what turned the teams off? Or were they just gun shy after the BRM V16 problems?
Perhaps some teams did consider supercharging but it is just not well documented?