> The periodicity does have to be a known compile-time ratio expressible as a fraction of seconds

Right, that's the problem I'm describing.

> or just pick one (or several) std::chrono base durations such as standardizing on nanoseconds, and then just count floating-point ticks.

None of this really fits well -- the idea is to count in integer ticks (invariant cycles), without doing expensive division or floating point math. It's like steady clock, but in ticks instead of nanos.