knorke: It is possible to put the metal spot textures anywhere inside the spring directory. For instance you could put them in /LuaUI/Widgets/MetalSpot/

it's a nice feature.
If your map is a desert map, you just write /LuaUI/Widgets/MetalSpot/desert_ on the first line of the .lmf file. That way it will search for desert_metal_high.png ... desert_metal_low.png in that location(in other words it supports filename prefixes too). This means you can have generalized textures. With the RGB tint feature, the mapper can also tint the texture to fit his/her map better.
The reason I didn't write about this feature is because I don't know how to deliver it to the users. Users would have to download an archive of general metal textures. The lobby clients only support downloading of the widget .lua file. Maybe it would be possible to release a 2nd version which comes in an archive that has general textures in it, and can be downloaded 1-click through lobbies. Do lobbies support downloading .7zip files in their widget downloaders?
Also that means the mappers need to endorse the widget and suggest some general textures for each terrain type that they want included.
GoogleFrog: I actually forgot about the save/load feature.

I had it in my mind to add it. It saves a lot of computation. As you say it would be great to have compatibility between different widgets using those metal maps. A simple routine to check if the file already exists, if not, then generate it, added to every widget, would assure compatibility. I'll be sure to include it in the next version.
knorke: This widget is not a manual-use kind of widget, it's just a passive widget that detects if lua metal spots should be drawn or not. As you say it's a big drawback to have to modify maps, there is no backward-compatibility. But do you have any proposal for how it is supposed to reliably determine if patches are to be drawn on older unsupported maps? And when it has done that, which textures is it supposed to use? A community metal patch texture set? If that is something desired then I can certainly try my best to create a guessing function, if I make it complex enough it can have decent results, it's just a question of if we want to go there or not. In my whole list of maps, I only could find one map that doesn't have metal spots: XantheTerra.
SirArtturi: I'm not an artistic type so can you please explain the alpha layer idea? Will it give patches can mend better into all maps? Proof of concept? Because to me it goes beyond just color, the shape of the patch could mean it fits into one map but not others. And then we also have the whole general texture delivery-issue again.
Does Spring lua include LuaSockets? If so then textures could actually be downloaded from a central server, solving the delivery-issue. Heck it could even automatically update textures that turn out to be unpopular.
Anyway it's good that everyone has thoughts and feedback. I want to take all your feedback and improve the widget, to make it as good as possible and meet everyone's needs.