You could try a node setup like this:
This will make the emission shader become brighter the longer a ray travels through the object:
However, this only works with a single object, it doesn't look so good with particles.
An alternative is:
By adding the transparent shader to itself:
#Update
With the new transparent depth output of the light path node, you can avoid getting black when you run out of transparent bounces, instead replacing the transparent shaders with an emission shader:
This way you can get away with lower transparent bounces, and not have annoying black spots.