To implement this functionality you have to modify FPGA gateware. LimeSDR-USB got 8 GPIOs you may use for switch control.
Another solution may be to utilize what is implemented in FPGA gateware 2.23 version already. FPGA_GPIO0 (J22, Pin 1) is used to indicate when data are being transmitted (high level indicates data transmition is active). To activate this you have to write to FPGA register 0x00C0 value 0xFFFE. Then feed this signal to Arduino and you will know when signal is being transmitted.
Hope this helps.