Meh. If quantum gravity could do it, then any other quantum force could do it.
I don't think we know anywhere near enough about quantum gravity to be sure of that.
Not that I'd be super-optimistic about "quantum gravitational computers" actually being any use relative to ordinary quantum computers -- but in the absence of an actual working quantum theory of gravity I don't see how we can know they wouldn't make a difference in calef's hypothetical world.
Speculation is important for forecasting; it's also fun. Speculation is usually conveyed in two forms: in the form of an argument, or encapsulated in fiction; each has their advantages, but both tend to be time-consuming. Presenting speculation in the form of an argument involves researching relevant background and formulating logical arguments. Presenting speculation in the form of fiction requires world-building and storytelling skills, but it can quickly give the reader an impression of the "big picture" implications of the speculation; this can be more effective at establishing the "emotional plausibility" of the speculation.
I suggest a storytelling medium which can combine attributes of both arguments and fiction, but requires less work than either. That is the "wikipedia article from the future." Fiction written by inexperienced sci-fi writers tends to generate into a speculative encyclopedia anyways--why not just admit that you want to write an encyclopedia in the first place? Post your "Wikipedia articles from the future" below.