Thanks for the great entry!
In order to make this work for higher frequencies, I would add Hartmut's suggestion of a frequency dividing input stage. If we divide the input down by 100, the overall range will be about 200MHz, and the noise will be about 20Hz/rtHz. That might be good enough... but we can hope that the commercial device is lower noise!
Last Friday, Matt made a frequency discriminator circuit on a bread board in order to test the idea and study the noise level. I think it will work for phase lock acquisition of Green locking.
As a result a response of 100kHz/V and a noise level of 2uV/rtHz @ 10Hz are yielded. This corresponds to 0.2Hz/rtHz @ 10Hz.
The motivation of using frequency discriminators is that it makes a frequency range wider and easier for lock acquisition of PLLs in green locking experiment.