however the author of Solar Bandits replaced the 'command_longrange_***' texture file with the 'cmd00.tex' file without simply renaming it. So there must be someway of editing the gui.
No, he didn't replace anything. He simply created a new command named "SpecialMenu" for command slot 12 in
universetacticsmod_commands.ini. Nexus doesn't know this command - so it doesn't know, which button texture to use for that. If that's the case, the game loads a placeholder texture named - you've probably guessed it - "cmd00.tex".
Try it for yourself and add something like
Single 5 "MyCrazyCommand"; to that file... you'll see the same "cmd00.tex" button texture ingame.
/edit:Forgot to mention: to include your own graphics for your own custom commands, you'll need two texture files:
command__on.tex
command__off.tex(in texturesiconscommands folder)
Of course, replace
with the actual name of your command in mod_commands.ini (for the solar bandits mod it would be "command_specialmenu_on.tex", for example)