So, instead I have a global variable that I access through globalAdjustGLMemory that I use to track how must memory DirectX would be expecting the textures to be using.
Yeah, this sounds like the hack there is in DDraw to make SS2 happy (basically, it allocated textures and checked that the space left was diminishing between calls and aborted if not).
The only problem now is to get the initial value on all supported cards in Linux (first to get the available space on the GPU and then, harder, the available free space once you have counter all the memory that X11 eats 'naturally' before even GL has started).
Lionel