I suspect the reason nobody makes a 52T-compitble chain guard is that the combination doesn't really make sense. A bike that has a 52T ring should get paired with spandex. Granted, older quasi-racing bikes are becoming city cruisers these days and bike manufacturers continue to produce bikes that look good on a spec sheet rather than bikes that have practical uses, so I can imagine how you might have a bike in mind that makes sense for a chainguard but has a 52T ring. I can't, however, imagine a situation in which you'd ever need a 52T ring on a bike that you were using in ways that made a chainguard practical.
Out of curiosity, what's the bike?