I didn’t check how good modexp gas schedule is, but one artifact is that implementations for odd moduli is wildly different from implementation for even moduli. And below a certain size, moving to Montgomery domain for Montgomery multiplication exposes you to denial-of-service.
That said, it’s “just” a matter of implementing and measuring the gas cost, one can do that in Constantine: EIP-2537 (BLS12 precompile) discussion thread - #77 by mratsim and even get detailed metering on internal calls, for example: constantine/metering/eip2537.md at master · mratsim/constantine · GitHub