I stated "The more bracing or thicker material used, the lower the frequency." This "bracing" refers to adding webbing to the lathe bed to change the frequency and increase stiffness.
Can't you weld/braze steel to cast with bronze? Speaking of materials development...one of the most specialized and interesting steel materials companies is Dynamic Materials (especially their Explosive Welding Division). This explosive cladding process was originally refined by DuPont back in the 60's and is now coming to the forefront for certain applications.
I think you mis-characterize the advantages of cast iron, especially given that many of these lathe designs were introduced so many years ago. I wouldn't agree that cast iron's properties have been embellished or hyped.
Maybe I'm splitting hairs but (please tune out here if you don't want to read techno-babble) ...I also think the material plays a bigger part than you mention. If it can attenuate better, then it doesn't have to be as massive. The irregular crystaline structure of CI slows and scatters phonons (vibration and sound) within the material due to impedance mismatches at the grain boundaries. This irregular structure tends to attenuate at a given frequency and scatter into other frequencies making the whole structure more capable of shock absorption without transmission. On the other hand... because steel is more regularly organized, it has better (more consistent) phonon transfer at a given frequency (less attenuation) and will transmit the shock better.
Apples to apples...I have 1" thick hardened steel plate (Rc>70) that will ring like a high-pitched bell but I've never found a 1" CI plate of the same size that would ring like that.