Coverage guided QuickCheck has been on my mind for a while, I believe it would greatly improve fuzzing experience and allow to easily and thoughroughly check/fuzz things that aren't parsers/decoders/deserializers or functions that operate on strings in general.
And yes, QuickCheck (or RapidCheck, for C++) is what you need if you enjoy manually specifying your test inputs. RamFuzz is a tool to support research into automating that and, eventually, the entire process of writing unit tests.
"Since Zalewski has already done the work of figuring out, empirically, what heuristics are likely to exercise more code paths, it seems like a waste to ignore that and just generate totally random values."
Dan does, in fact, explicitly suggest "using fuzzing-like mutators", as you put it, in QuickCheck's random value generator. But those mutators work on continuous file contents, not on a bunch of disparate parameter values generated at distinct moments in the program. An early version of RamFuzz tried to apply AFL to the log file recording all generated random values, but all that yielded was invalid logs that couldn't be replayed. I don't know how else one would leverage Zalewski's heuristics in a random-value generator, though I'm obviously open to suggestions.