RISC OS 5 IOMD ROM simulated IIC bus?

Using today Beta IOMD ROM

IIC_Control finds a – simulated? – IIC device at address &50

What is this – simulated RiscPC port expander? RTC? Could something useful be done with it?

On a RiscPC (and all(?) Acorn machines prior to this), the IIC device at &50 is the RTC/CMOS chip.

You are right, it is this one, reading address &A1 means &A1>>1 = &50

https://www.nxp.com/docs/en/data-sheet/PCF8583.pdf

I will try if it is – fully – simulated by reading its registers.

Yes, that’s the one, the PCF8583 is the chip Acorn used right up to the RiscPC.