After some more digging, experimentation and consultations with other members in the community (thanks to BrowseMan on the RetroWeb Discord channel and fellow Vogoner mkarcher) I have successfully managed to disable internal keyboard controller on the UM8498F chipset and was able to install an external one with PS2 mouse support, at least on my motherboard Octek Hippo 10.

The internal KBC itself is integrated into the UM8498F chip but there is some logic in the smaller UM8496F which needs to be addressed too. Let's start with pinouts.

UM8498F
pin 119 --> internal KBC enabled/disabled (pull-down disabled, pull-up enabled)
pin 165 --> chip enabled signal for external KBC and BIOS ROM in both modes
pin 166 --> KB data signal in internal mode; direct connection to CPU reset pin in external mode
pin 167 --> KB clock in internal mode; keyboard controller clock output in the external mode

UM8496F
pin 4 --> internal/external KBC mode (the same rules as pin 119 on UM8498F)
pin 20 --> inverter 1A (MSDO on my board)
pin 21 --> inverter 1Y (MSDI)
pin 22 --> inverter 2A (MSCO)
pin 23 --> inverter 2Y (KBDI/MSCI)
pin 26 --> inverter 3A (KBCO)
pin 27 --> inverter 3Y (KBCI)
pin 28 --> inverter 4A (KBDO)
pin 30 --> inverter 4Y (KBDI in PS2 mode)
pin 44 --> A20 signal in
pin 45 --> RESET signal in

On many boards there is a jumper that should be set/unset in dependency on the chosen KBC mode. On my board, it connected KEYLOCK frontpanel pin to UM8496F pin 45. This jumper should be removed when external KBC is being used (pin 45 may have a different function in internal KBC mode).

I hope this will be helpful for anyone wanting to mod UM8498F/UM8496F motherboard for PS2 mouse support.