In a way, you can use LAVA to store MJ energy.
With a Crucible from TE you can use 20,000MJ to melt down 1 block of cobble into one bucket of lava. Store as much lava as you like in the giant iron tanks thanks to railcraft.
The lava can then later be used to power a magmatic for the same amount(assume the magmatic engine is warm). So if your looking for a place to dump extra MJ that is not being used...LAVA is a good bet.
Bonus 16k MJ in this conversion to lava if you throw netherack instead of cobble into the crucible.