X and SteamOS

I don't think it is wise or ethically correct to ban specific technologies from steam. Any by the way, with this argument they won't need LD_PRELOAD either. They can just provide their own implementation of the X-Protocol and enforce use of that.

> That sounds a bit complex to handle the interception of a few
> functions

It's really not. I would actually argue, it is a *lot* easier then doing an LD_PRELOAD mechanism in a portable and stable way.

> not to mention that the request still have to go through some
> ICP mechanism.

You add a roundtrip through a UDS or a loopback device, yes. That won't make any practical difference. I am not entirely sure about "a few functions" either, by the way, depending on how they implemented the rescaling. But they quite possibly might have to rescale on pretty much *any* function that handles coordinates, which would mean pretty much any function. With triplication for Xlib, xcb and the serverside-fallback. And since Xlib has such a non-uniform API you would actually have to touch all functions it provides.
With a protocol-interception you can generate code for all X-request in an automated way (e.g. from the xcb protocol descriptions) and handle in one catch *all* ways to communicate with the X-Server.