It is indeed possible to do so. The
Simple Directmedia Layer is a frequently valuable code resource just for containing so much generic code covering so many different platforms, and one of the things it does is check an environment variable to see if it should alter where it puts the window...
Here is the source code for
SDL_x11video.c, please check out the code in X11_SetSizeHints, it seems to use XMoveWindow among other things.