Specifically, I tried setting the wood gas fluid type in a Refined Storage exporter, and the wood gas cell won't work. I assume this is because it's not registered as a fluid container?

It is registered as one.

IC2Fluid.createFluid("woodgas", new IC2Fluid("woodgas").setDensity(-1000).setViscosity(500));
FluidContainerRegistry.registerFluidContainer(FluidRegistry.getFluidStack("woodgas", 1000), Ic2Items.woodGasCell.copy(), Ic2Items.emptyCell.copy());

It could be that it is because it doesnt use the newest system of Forge and still using a older system which is better for general support.
It could be refined storage only support the new system. Try lava & water cells if they work no longer.

Yup, water and lava cells don't work, either

Yeah basically RS forcing in the new system. Which has a lot of issues on its own thats why i dropped it.
I wonder why it doesnt support it since forge has a backport for that kind of system.

closing because added to a todolist