I've built and tested a prototype of the XMega32E5 controlled OH300, and it works fine. This has allowed me to return to a single firmware source for both boards. In the source code, you include an oscillator include file that has a bunch of macro definitions to tailor the source for that oscillator. In particular, the two board types are differentiated by the SPI_DAC macro. The FEI boards do not define that macro, and the OCXO and TCXO variants (not sure if I will go to the trouble of designing one of those - they don't really sell) do. In addition, if SPI_DAC is defined, you can choose between the AD5680 and AD5061 DACs depending on whether you need 16 or 18 bits of resolution (16 bits is for the TCXO variant). Next, the oscillator control file is where you define the EFC GAIN and (for the FEI board) any BIT_REDUCE value required to bring the gain down to a useful range. Lastly, the file defines the two or three time constants to be used by the modes. Longer time constants trade phase control for frequency stability.
If your oscillator has an inverse EFC slope (higher EFC means lower frequency), then you can change the DAC_SIGN value from 1 to -1. Other than that, there should be nothing in the actual source code to change.
The source repository includes support for the FE-56x0A, FE-405B (using the FEI board), and the OH300, ECOC-2522 (using the OCXO board) and the DOT050V (using a TCXO board, but I may not bother designing one).