According to Alton Brown, talking about why a short-grain rice salad won't get hard when cold:
But it won't be hard, the way, say,
refrigerated Chinese take-out
restaurant rice would be. Why is that?
Well, long-grain rice, which is
usually what's inside one of these,
contains a very, very high percentage
of amylose. [holds up the model]
Remember this guy? When this cools,
the amylose and water come together to
make a structure that's kind of like,
well, kind of like a crystal, which is
why it's hard as a rock. The process
is called "retrogradation", and it's
reversed when the rice is reheated.
Now since medium-grain rices have more
amylopectin in them, this never
happens in the first place, which is
why I use medium, and sometimes
short-grain rices for all
If you can't heat up the rice, have you tried making some sort of short-grain variety?