Does each sensory neuron type have a characteristic spike sequence pattern?
This is an interesting idea, but I do not think it's correct. One piece of information that goes against the idea is this: auditory information is encoded by both frequency and amplitude modulation of neural spiking. The idea of spiking rate directly correlating with frequency is at odds with the idea of spiking rate containing this sort of "meta" information about the source of the activity.

We know quite a lot about how sensory information is represented in the nervous system. The simple fact of myelinated axons that don't interfere with each other already accounts for the problem you've proposed a solution for. 


  if all 3 types of cone cells generated identical responses for a given rate-of-incidence... then there would be no way for upstream neurons to tell what type of cone cell a spike sequence came from


Here I believe is where you've made an error. The transmission of activity from the sensory neuron itself to an upstream neuron inherently conveys information about the sensation. The firing of the sensory neuron is a translation of the external phenomena to an internal neural code. No extra information is needed to represent the source of the activity.

References

Li Liang, L., Lu, T., &amp; Wang, X. (2002) Neural Representations of Sinusoidal Amplitude and Frequency Modulations in the Primary Auditory Cortex of Awake Primates. J. Neurophysiol 87:2237-2261.