Replying to myself. Ok can set the LED on/off with:
AT+SYSREG=1,0x60091020,0x8001
AT+SYSREG=1,0x60091020,0x0001
Note: Keeping bit 1 set each time because GPIO1 is set as already for UART-TX
However, here is where things get odd. By default the XIAO running ESP Firmware seems to already use the external antenna? Booting with default firmware - not changing any registers - and scanning with AT+CWLAP indicates that its using external antenna by default (ie. drastic difference in reception when removing external antenna)
Is that a known thing?? Perhaps Seeed chose same setup as Espressif already used so ESP AT firmware by default enables external antenna? Any theories out there?