The open chamber dual-fuel engines our company has built operate at about 4% pilot fuel (diesel) and 96% natural gas. The diesel fuel is only used as an ignition source (like a spark plug) for the natural gas.
With pre-combustion chambers, we can get down to 1-1. 5% pilot fuel with significantly improved emissions.
When operating in dual-fuel mode, however, these engines regulate intake air with air/fuel ratio controls just like a spark-fired engine. Otherwise, detonation and misfire can occur.
Whistleman, please be aware that some of the ETH owners have reported head gasket failures on propane.

This is speculation only on my part, but the higher compression ratio of the ETH will result in higher cylinder temperatures before diesel injection starts, resulting in a greater tendency for the propane/air mixture to auto-ignite, assuming one is running "heavy" enough propane injection to produce a combustible fuel/air mixture. Also, the ETH will have higher peak firing pressures with propane than the ETC, so this doesn't help the head gasket situation.
The sulfated ash & phosphorus content of lubricating oil for gaseous-fueled engines is critical because ash deposits in the combustion chamber can act as a glow plug and cause preignition. Natural gas engine oils for 2-cycle engines typically have sulfated ash levels of below 0. 1% - 4-cycle engines may require more ash to prevent valve/seat recession. Phosphorus, typically in the form of zinc dithiophosphate, an anti-scuff/anti-wear additive, is more of a problem with spark-ignited gaseous fueled engines as it tends to build deposits that foul spark plugs.
Rusty