The mcu itself leaves ram alone.
Well, you wouldn't want to jump to 0 anyway, but the problem with jumping to the reset vector is that the rest of the processor is not reset.
Whats more you would still end up going thru the "C" runtime start up code.
The "C" run time start up will set global variables to there declared initial values.
If no initial value is given they are set to 0 by default.
It is the "C" run time that calls main.
The way to handle this could be to exclude a section of ram so that the complier and linker will not put anything there, then set a pointer to it. I don't use Cosmic, so I can't tell you how. If you used CW, you could adjust the PRM file for this. This section of ram will then not be set by the start up code.