There's performance overhead to doing everything, but one way to simulate zero'd virtual memory is to simply map them to a zero page and, when a full page write occurs to simply write the page, and when a partial write occurs to zero the rest.

I am not familiar with GPU internals enough, but my understanding is that the GPU should be smart enough to know that a given texture or framebuffer will occupy n full pages, and so when either is written in its entirety, the zeroing only has to occur at the edges. (I would assume that the write would start on a page, but I don't know anything about GPU internals.)

Caveat emptor: I will reiterate I know very little about memory internals. It seems like a bigger issue is that GPU memory is not virtualized and all users get access to the same memory. It's as if three decades of understanding the utility of virtual memory were forgotten.