Openwrt is designed as a complete OS, and would fully replace everything on the original device except for bootloader, bootloader config, and sometimes WiFi firmware if that’s provided under a separate partition. So no , not possible to run it under original kernel. But to be honest, it’s usually so old you probably shouldn’t anyway due to security issues.
I’d suggest just posting photos and your analysis on this thread right here. If you take some high res photos top and bottom of your board I might be able to identify the serial pinouts .