Many modern Realtek chips still support RRCP. You just need to enable it. For example for RTL8370N: Register 0x18d6 configures the 16-bit key, 0x18d4 selects which ports can use it (0xFF for all, normally only the cpu port), 0x18d3=0x1 enables it.

You can write these registers via the management interface or via the EEPROM. It will not respond to discovery packets, but get and set packets work fine.

This chip also has a 8051 core that can access the internal bus and can tx/rx network packets. To use it you either attach external SPI flash (large program) or write the program into the internal RAM in the chip (small program).

All documented stuff, no secret details.