JEI now has support for custom ingredient types, in addition to ItemStack and FluidStack.
I think Mekanism's gasses are a good candidate for this feature.

Here's the wiki documentation on adding new ingredients: https://github.com/mezz/JustEnoughItems/wiki/Non-Item-Ingredients
The vanilla plugin in JEI adds ItemStack and FluidStack using the same API, so you can look there for an example.

Let me know if you have any questions.

Maybe doing it the other way and changing Mekanism gases to use the regular fluid types in Forge would be a better idea, though that might be a lot more work.

Have their been any further thoughts on doing this? It's hard to remember how to make Sulfuric Acid without consulting a wiki.

I'm willing to pick this up, together with an update of the used JEI api as soon as my 1.12 update and 1.10 back port are accepted and merged.

This appears to already have been added in 89905c8#diff-055b8be0fe0984f3a1efe9ff4ff5173cR162 ..

Completed fully here: 7b9bf21