You can see how at line 72 there is a case statement and if the TX
FIFO is empty, the values are set to be zero. I am not 100% sure, but
I believe if you change these values to be non-zero you will get a
carrier to come out.
Moreover, once the TX FIFOs are filled with something, that data will
then automatically switch over for you.
The only thing I'd be worried about is the master control register
which controls whether or not the TX strobes are enabled. You can see
this here:
http://gnuradio.org/trac/browser/gnuradio/trunk/usrp/fpga/sdr_lib/master_control.v
Follow the 'enable_tx' signal around to see what I mean.
You may want to dig into Verilog simulation a bit if you have the
time. There are testbenches that are written and in the repository
that allow you to see what the changes you make will have on the code.
I hope this helps.
Brian