I've got an unusual use case (optical communications) that would benefit from access to the wifi baseband signal before it is upconverted/modulated at 2.5Ghz. However most modern wifi chips combine baseband encoding and transmission encoding/transceiver functions on a single chip. Are there chips, or chipsets, that split these functions and would allow for custom final encoding - in my case OOK laser diode modulation at << GHz frequencies?
Your request makes no sense - WiFi baseband is OFDM, and can't be converted to OOK.
None of the things that WiFi does to synchronize would even work with OOK.
It seems to me that you just want the bits going into the modulator. If that's the case: you don't need any hardware - that is exactly what your operating system sends to your WiFi card, and you need just a tap device (Linux networking terminology) to give you access to raw L2 packets.
Based on these, write software that controls youe light source. Later, port that to become a proper network card device driver.