Apart from using the MIB counters on the BCM5328x devices, are there any other internal registers / flags to indicate network activity on the FE or Gigabit ports which could be used to drive LEDs, rather than using the LEDDATA and LEDCLK serial bit-stream and associated shift registers (which uses up quite a lot of real estate on a PCB?).
Per your questions, no any internal registers.
5328x supports serial LED mode.
Suggest to follow 5328x datasheet LED Interface section and design guide application note Section 7 for the LED design implementation.