The main problem is that Unreal doesn't support dynamically loaded textures as far as I know, as in you can just place a TGA or DDS into a folder and then Unreal will load it on command. Even so, placing it into a UTX file can cause problems as the MD5 will differ to the server.
Also if clients had their own textures how would you make sure that all the clients would see the correct image? That would mean that the texture would have to collected by the server and then the server would distribute it. That could be rather dangerous and unwanted by clients and servers.
Lastly, the only way I know of doing this is using a UScript texture. Get the client to design his own spray paint by using mathmatical equations. Sure if won't look as pretty, but it would mean that the data that this client transfers are maths equations which would result in a small packet size. That might work, but it is only in theory.
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