This is an interesting problem. I do not have a good suggestion for you. I have asked several people the question, and they also do not have good answers.

Perhaps if we look at the septic tank and drainfield as seperate problems, we can get some ideas. You say that the homewoner is not using anything special or overloading the system. Can they be using a lot of cleaners or other organic materials? The suggestions that the organics are staying in suspension might indicate an organic grease dissolver type of material. It could also mean that something with a lot of surfactants is being used. This could include things like shampoos or cream rinses. These materials do not seperate well in the tank, especially if the tank is small or the flow is great. Look for something like these to help determine what is happening in the septic tank. This may be helped by using a professional bacterial additive in the tank.

The description of the drainfield sounds like a typical failure due to clogging and buildup of sulfides. The black jelly like material sounds like sulfides and the gray skin the oxidation product of sulfides and the oxygen in the air. This clogging of the field can be due to overloading due the the orgainc material passing through the septic tank and increasing the BOD demand of the drainfield beyound its capacity to stay aerobic. 

Assuming that the drainfield was constructed properly, Septic-Scrub may help here. If we can get some flow restarted and then follow with a shock dose of professional bacteria to consume the excess bacteria, we may be able to restore the drainfield. 

I do not know if this helps, but I am interested in what happenes to the system. Please call to further discuss this situation further.