I would personnaly be against leaving a mark inside a mod explicitly. Brand usage is a good idea albeit I would personnally never use brand clothes that have the it all over the backside nor would I use a body that has a watermark under the left foot of my character. Characters that are action figures are just not my thing. Such things should be best stored in metadata or in readme files. Also modders work should speak for itself anyways.
This answer is aimed at @tomomi1922
Textures and images fall under the copyright law. Meshes would fall in Europe under Industrail design rights and in the USA either under trademark or copyright law. Copyright protection comes into existance by the whole creation of the artwork itself. It does not need special registering and usually falls under the copyright law. I would certainly not trust that photographer's legal counsel. The best you can do is go to a notarius and get certified that he saw you with this exact object at that date. This is the law that lasts 75 years after author's death.
If another person uses your texture and adds enough original expression within your work it is no longer considered derivative therefore stands in its own right. In case of textures the limit is very very low to get into the creative expression barrier. How many cases of legal disputes have you seen due to in game textures between publishers? The modification is not required to be very important. Obviously the copyright infingement is tested for viability in court. As well as copyright can be challenged in court.
Meshes on the other hand protect the whole design element however not individual parts of it. As seen in prior cases if I suddenly started producing a car that has the front of BMW 7xx and the behind of Audi A8 obviously I would not be infinging upon the design rights of either Audi or BMW or both. Obviously design rights have to be requested and registered at the relevant authorities as well as paid for, for the requested duration.
And obviously it is the responsability of the copyright holder to verify if there is any infringment of his copyright. Unless the copright holder himself claims the infringement no third party may intervene by exerting the rights that are only his to exert.
Also there is no registry of all copyrighted work because it is just not needed nor possible to have.
I thought it is interessting to point this out since many people talk about copyright, trademarks, patents and so on but do not necesseraly know what is hidden behind it. Even in photographer's forums one can see impressive misconception to the "width" of the domain that that they percieve they are protected from.