The UHD drivers allow for additonal arguments for streaming applications using the --args option (https://files.ettus.com/manual/page_transport.html#transport_udp_linux):

rx_samples_to_file --duration 60 --file /tmp/data.dat --rate 30000000 --args="num_recv_frames=5000"

For applications that use SoapySDR, is there a way to pass these additional UHD specific arguments through to the SDR using the Soapy API?