It seems 14th Gen CPU is already working on Z790 with the latest LAST BIOS as per a posting from someone at the overclockers website:
Post is by: Talon2016
Quoted:
14900K up and running fine on my Z690 Dark on current BIOS 2.13.
BIOS recognizes the CPU as 14900K, and clocks and boosting all appear to be working as intended.
URL: overclock.net/threads/evga-dark-k-ngp-n-z790-owners-thread.1804023/page-30