I don't need them for thermal imaging but when filming video I am forced to use too fast shutter speeds. I have read the for best results it should be 24fps with a 60 shutter speed (closest to 2x the frame rate). Unless it is quite dark out, this is not possible.
If you read this thread you will be provided with an alternate viewpoint on ND filters.
Why I NEVER use ND Filters with Drone Cameras. And if you read this thread you will also be presented with an alternative viewpoint on shooting at 24FPS.
Why I NEVER Shoot Video at 24FPS you will then see why I said you probably don't need them.
I also wondered about having ND 8 and/or ND 16 filtering available in order to reduce video flickering due to oscillating prop shadows by using longer exposure times. This will "stretch out" the shadow phases. For longer exposure times according to the "180° shutter angle rule" it would also be technically possible to use lower or "negative" ISO values.
Maybe an "electronically induced ND filter" (like i.e. in the FX9 camera by Sony) will become a more common feature also for the smaller chipset cameras...
BTW the RGB camera of the Dual 640T is really great and powerful!
I have some very lightwighted ND filters for a Typhoon H CGO3+. Maybe there is a way to place it i.e. with a tiny ring of velcro...
The solution to your flickering issues as well as additional information on why an ND filter is not needed is also provided in this thread:
Why I NEVER use ND Filters with Drone Cameras. The short answer to your flickering problem is either get/make a lens hood, or rotate/tilt the camera/drone until the flickering is gone. Attempting to lower your shutter speed to what is potentially a very low number which will add undesirable softness to images is not a good solution not to mention for video you would be limited by the video's framerate meaning you wouldn't be able to go below 1/24s for shutter speed anyway. Even if 1/24s was slow enough, it would be unlikely to produce reliable results since the blades rotational speed changes based on what the drone is doing at the time. It is much better to just prevent it from happening in the first place (lens hood, camera's relation to the sun), than try to fix it with a filter.
As far as "negative" ISO values goes, there is no such thing. Cameras have one or more ISO values baked into their sensor circuitry by the maker; for most cameras that is ISO100 unless shooting in a LOG profile which could push it up to something like ISO800. ISO is simply the amount of circuit gain applied to the incoming signal. It is possible to go below the native ISO if the software/camera lets you. For example, my cinema camera's native ISO when shooting in a LOG profile is ISO800; I can go below it to ISO640 or ISO400 but it is still not negative. Since ISO is a function of circuit gain, it is not possible to go below zero which would result in no signal at all.
I have in the past dropped to ISO50 with my regular camera to get more motion blur for something like a waterfall on a bright day when the ND filter was not strong enough. I do agree that it would be great to see more cameras come out with eND technology; no idea why Sony is still the only one with this. None of the latest cameras from other makers have it; possibly due to production costs, Sony patents, or something else that we are not privy to.