You don't need to refresh the DRAM if the memory’s not being used. The trick would be figuring out which memory is unused, or could be swapped out to save power. Since memory controllers are (becoming) integrated into the processors, that's not so infeasible. (And of course, libc needs to be capable of deallocating core when pages in the heap become empy.)

And why shouldn't hard disks be capable of working at 50% speed? Why should they have to go flat out, every time I want to write a line to a logfile.