USBOpen(), USBSend() and USBRecv()

[gehakte_bits]

See the help text of USBSend:
USBSend({data}) is the format to send data using the HID interface.
i.e. USBSend({240, 4, 65, 66, 67,68})

Per the FT260 User guide manual
4.6.2 UART Write Request:
Byte 0 0xF0 Report ID (240)
Byte 1 length (04)
Byte 2..5 actual data 'ABCD' (65,66,67,68)

A PC typically expects RS232 levels, sometimes 0~5V will do, so I don't know if 0/3.3V will be properly recognized. You might need a proper RS232 level converter.

ft260_EEPROMrdall() dumps the FT260 internal EEPROM (#C2h I2C Read Request), it is not using the serial port to your PC.
The data you see is from the FT260 HID buffer, after a USBRecv(). The first byte indicates what port it came from (probably 0xf0 up to 0xff in your case as from the UART)

Good job - you are almost there!