I am developing a mod for minecraft 1.12.2 that among other things adds a bunch of biomes to the overworld, and I have recently discovered that said biomes do not appear in BOP worlds. I've been digging around in the biomes o'plenty source code and the api, but I could not find any logical way to make my biomes generate in the BOP worldtype. Is there something I'm missing here or is it simpy not possible to do this?

See what other mods do? There are a handful of other mods (That I can't name off the top of my head) that add biomes that work just fine with our world type.

If you're registering biomes with Forge's standard BiomeManager they will appear in our worldtype just fine. https://github.com/MinecraftForge/MinecraftForge/blob/1.12.x/src/main/java/net/minecraftforge/common/BiomeManager.java