well it was exclussivly for gp4.gamer.nl carset as you say, but as that has been released everybody is free to use the same method imho, we just wanted the set to be the first one ever to have this. so i'll give advice and help where can
you can edit these values in the exe with a so called "hexeditor" there are many out there, just do a google search and you'll find several. you can use either one.
You can spin them round as much as you want with the special tweak. But I doubt an upside down helmet adds to the realism of gp4 When using the patch.ini you have the max movement from the helmet. I think for the mods that are in progress it's better to use the tweak so you can see the wheel turn a bit more.