Unfortunately...NO, it is not possible to rubber mount a pre-04 Sporty. The only way to do it is to use the engine to do a complete custom built bike using a rubber mounted aftermarket frame, the total job would cost well over what you are paying for the Sporty, around $15k if you want it done right.
It is commonly known that rigid mount HD's do vibrate, but IT IS NOT THAT BAD!!!!....Have you ever ridden an Evo Sporty?...They are actually one of HD's lesser vibrating bikes. I have known countless people over the years that have steered away from HD because of this myth, but once they finally rode one, the majority were surpised that the vibration was not that bad at all. If the bike is PROPERLY maintained....meaning SERVICED, not just an occassional oil change, they will last longer than most imports.