You can use the layer-emitter for that. Particular will look for the alpha and spawn more particles where areas are more opaque. Just make sure that your text-layer is precomped and set as a 3D-layer. Then, from the drop-down menu of particular, choose the layer-emitter, open the tab of the layer-emitter further down and select your text-precomp. You might wanna up the particle count as thin text may not cause a single one to spawn, and you will think you did something wrong. Alternatively, use the layer-grid-emitter to significantly up the particle-count. Just make sure to disperse the particles using wind or Spin-Amplitude down in the physics->Air tab.
I hope I was able to help you, have a great day!