I am rebuilding a double overhead V-6 engine and have noticed wear on the bucket cam followers and would like to know if anyone has had a good result with DLC coatings applied to the cams and or followers? 

This technology is beginning to be used in mass produced engines.  Nissan and Caterpillar are both actively working with DLC coatings.  There are a number of SAE Technical Papers documenting some of the recent research, implementation, etc.  The annual conference of the Society of Vacuum Coaters is another source of information on automotive applications.

If designed, manufactured and maintained properly, a cam and bucket follower interface would not benefit from an (expensive) DLC surface treatment.  The reason is that when the parts are properly designed and manufactured, the contact between the bucket and cam lobe should always be hydrodynamic (ie. the parts separated by a thin dynamic oil film), except during start-up and stopping.  As long as the cam lobe and bucket surfaces have adequate compressive fatigue strength for the magnitude and number of load cycles it must endure, based on the hydrodynamic oil film pressures in the local contact zone, then the parts should have adequate life.

Adding a very thin, but very hard, DLC surface won't really help fatigue life either.  The classic failure mode for this type of structure (ie. a thin hard case over a softer core under high localized contact stresses), is case spalling due to sub-surface initiated shear fractures.  What this means is that it does no good to have a very hard, high strength case unless you have a core material with very high shear strength to back it up.

DlC's are great due to their hardness and low inherent friction properties.  And they have their place when utilized properly.  They are most beneficial for boundary-type contact conditions, where oils or greases cannot be used, or where a loss of lube may occur.  But an engine valvetrain bucket follower and cam lobe would not likely have these operating conditions.  So save your money.

You mentioned one of the prime reasons to consider DLC(gate valve) - low friction.  To increase engine efficiency, low viscosity lubricants are used.  This means the metallic surfaces that move will be more likely to wear, so a wear-resistant coating improves part durability.  Also, the low friction reduces powertrain losses for reduced fuel consumption.

DLC coatings are applied by vacuum processes (PVD or PACVD) with a thickness of ~ 2-5 micrometers on surfaces that have already been highly finished (combinations of grinding, polishing, etc.), usually with a roughness of Ra ~ 0.1 to 0.2 micrometers.  Maximum roughness should be Rz ~ 0.4 micrometers, otherwise the coating essentially acts as a microscopic grinding wheel and degrades the mating surface during operation.  Anyway, DLC coatings do not require subsequent finishing when the substrate surface has been properly finished.  "Restoring" a previously worn surface is not really practical, because the surface is too rough (> Ra ~ 0.4).

"EP additives like ZDTP work well in gear oils, but they have properties that are detrimental when used in engine oils."

If this refers to Zinc dialkyldithiophosphates it is not completely accurate.  These compounds are classical antiwear additives, generally accepted as forming complex phosphate-based glasses on iron or steel surfaces such as cams and lifters under conditions of moderate load.  Some form of AW is necessary for most engines to protect the valvetrain during start ups, and zinc compounds serve this purpose in nearly all current engine oils and most hydraulic fluids.  They also function as anti-oxidants, protecting the oil from high temperature chemical degradation.  I don't recall how they interact with DLCs, but this has been extensively researched so there is literature on the topic for those interested.

Extreme pressure additives are usually based on sulfur, forming iron sulfides under conditions of high load found in the low speed sliding contacts of hypoid and bevel gearsets in axles.  They are different chemicals suited for separate applications.  They are not generally suitable for crankcase oils, part of the reason why gear oils have different SAE viscosity grades despite comparable viscosity values (in some cases) to engine oils- to avoid misapplication.

As I have pointed out elsewhere, AW additives function at very low levels- consider hydraulic oils can get by for thousands of hours on ca. 50 ppm-  and the majority present in engine oils is included for oil durability (i.e. long drain).  In cases of premature engine wear one would expect packing the oil full of more ZDDP to have less real effect than making a parts change or selecting a different ZDDP chemistry better 'tuned' to activate at a more suitable temperature/pressure regime.

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