It's possible to add a modded fuel by patching the \interface\mechfuel\mechfuel.config file, but the fuel gauge that displays above the mech while inside it is hard-coded in the \scripts\deployment\playermechdeployment.lua file (specifically in the drawEnergyBar function) via a chain of if ifelse checks

I propose to add a config file (probably called "mechfuelbar.config" or something in the same folder) that contains an array of types with each element of the array having a type, frame, and bar, then have the lua check via a loop (while, return until, or for loop) that once it finds a match, it assigns the frame and bar to the respective strings in the config file, then use a break to break out of the loop, so you don't have to check the rest. The default "/null.png" would be assigned before the loop, and simply overwritten if found (if not it stays the same.

I would do it myself, but this is your mod, and I don't have much experience in writing in Lua.