I've decided to try one more pass at improving the sine wave output. This will involve ganging two of the NB3N551 outputs together to improve the output drive (ditching the square wave output in the process), and using a coaxial output - a U.FL jack on the board and an SMA pigtail to the front panel.
I anticipate that the result of this will be a very, very clean output with reduced load change sensitivity.
This change can't readily be done on the FE56x0 board because it also supports the FE405B, which has an output frequency of 15 MHz. On that board with the 405 oscillator, the LPF suppresses the sine output entirely because the corner frequency of the LPF is too low. In principle, you could alter the components of the LPF to raise its corner frequency, but I'm too lazy.