Just a thought. I have heard MJ referred to as pneumatic (Air) power. If we think of MJ in this fashion, then would it not be plausible to add an "air tank" to BC for MJ storage? After all in most large pneumatic systems the motor that compresses the air is generally a separate unit attached to the storage tank. Different motors use different fuel sources such as electricity and gasoline. Having worked in factories IRL that use quite large pneumatic systems I have seen a fair few different setups. All have one thing in common, storage for air pressure. Would not a MJ storage device be similar to an air tank? On that note if such a thing were to made a part of BC perhaps it could mirror real world pneumatic systems in the need for a MJ threshold to be met, say 100MJ.
In this case if the stored MJ dropped below 100MJ then the connected equipment (quarry, filler, forestry multi-block farm) would no longer continue to operate. Such a system would also introduce a "ramp-up time", a period of time where the engines would need to run before anything could be powered as is the case for real world pneumatic systems.
Again this is just a thought I had. Not one intended to start any fights about which mod has the better power system or something else.