Cgminer RPC API is a good place to start (https://github.com/bitmaintech/Antminer_firmware/blob/master/sources/meta-antminer/recipes-bitmianer/cgminer/cgminer-1.0/cgminer-3.12.0_bak/API-README). Luxos has some pretty good docs on the Cgminer API as well (https://docs.luxor.tech/firmware/api/intro).

Custom firmware would help, but it may be tough to find one for some of your less popular Antminer models.

If you have someone with engineering experience it may be possible to reverse engineer the APIs used for the firmware's local UI dashboards.