The solution is to put the DAC in Mix Mode.
Added one line of code to the "DAC Init" portion of the function "FMC230_dac_init" located in the source file "fmc230_dac.cpp" in the software reference design for the FMC230.
The line of code is
rc = sipif_writesipreg(bar_dac+0x19, 0x01); Sleep(2); //Enable Mix Mode to operate in second Nyquist Zone
Results as observed on a spectrum analyzer:
freq. (MHz) Nyquist Zone Amplitude (dBm)
100 1 -29
1233.333 2 1
1433.333 3 -2
2566.666 4 -43