Skypilot, I have had the same quest on finding the Boeing flight test frequencies. I wrote about it a few years ago in The Spectrum Monitor magazine.
I was able to check several older military facility directories and it does list several specific VHF and UHF channels for the Boeing plant. The facility is officially listed as “Boeing Helicopters Center 3 South”. Here is what I have found in various documents, but have not been able to confirm they are in use:
5.575 MHz LSB
13.922 MHz USB
32.1000 FM
40.9000 FM
123.2000
123.2500
123.4500
267.2000
321.7000
366.5000
In addition to these specific frequencies, here is a list of known Boeing flight test frequencies that I keep an ear on:
123.15
123.25
123.225
123.325
123.35
123.425
123.475
123.55
239.85
239.9
240.0
252.2
257.475
257.525
262.5
272.535
277.8
287.2
291.8
292.1
299.95
300.0
341.6
349.6
355.6
361.5
363.325
363.35
365.0
367.6
Good luck and let us know if you catch anything at all!
- Chris