What I mean by the phrase "spring-loaded firing pin" is a firing pin assembly similar in function to that of the M1911 semiautomatic pistol where a cartridge may be safely loaded into its chamber while the hammer is fully down. The slightly too short firing pin is driven into that cartridge's primer by hammer-and-hammer-spring overcoming firing pin's retracting spring.

Okay, to my question. Is such an after-market assembly available for Rossi Model 92 or Winchester Model 1892 lever action rifles that allows the rifle to safely have its hammer down with cartridge in its chamber?

It sounds like you are referring to a transfer bar assembly. To my knowledge there are no aftermarket upgrades. Newer models and repro's generally have a safety feature.

You're describing a kinetic firing system. The hammer bottoms out after contacting the firing pin and the firing pin continues on it's own under kinetic energy before it actually strikes the primer. The primer is detonated with stored energy rather than a direct transfer of energy between hammer, firing pin and primer.

As to your question, I'm inclined to say no. Those guns were designed with half cocks as a mechanical safety.

Yep, one of the best features of the Rossi is the half cock feature.
In regard to the firing pin, it has no spring in these models, inertia alone drives it forward and a notch in the firing pins shaft stops forward travel by a cross pin.
When at rest, the firing pin is free to move fore and aft when the hammer is cocked. I wouldn’t want the firing pin resting on a loaded round this way.

Cheers.