Snow/ice will certainly reduce the friction on the stone and therefore make it easier to drag. However, there are two downsides: firstly, there is less braking friction, so the stone is harder to control on slopes and secondly, the friction under the feet of the haulers will be reduced by a similar amount (I haven't heard of any Bronze Age crampons being discovered) ;o). A ramp woul dhave to be constructed from solid ice blocks. I don't think heaped up snow would be strong enough for a 40 tonner.

Nice idea, though.