I don't quite understand this. Depending on what body/flash you use, an AF assist light is available.
1) CDAF requires constant illumination (as it is sufficiently iterative process), most (all ?) external flashes with AF assist do strobe some pattern to assist off sensor PDAF - can't be really used by CDAF, as to why cameras with on sensor PDAF can't use that, no idea - but may be it is still too weak illumination for on sensor PDAF to work and then even when it works some final focusing still CDAF based (hence still need constant illumination during that process)...
2) some external flashes have LED video light too, but you are not seriously going to use that for CDAF assist as it is very irritating, even more than PDAF assist strobes...
3) on board CDAF assisting light is very weak, easily got blocked w/ some lens/hood combination, etc...
---
so somebody must make an external flash with a normal CDAF assisting light which is less irritating that video LED (plus I am not sure that video LED is flashes, like m43 FL600* for example, can be automatically switched on/off during CDAF focusing process - some owners can answer).