I suggest that your main concern will be with the vibration characteristics... why not do what we all do ? .. and put the camera somewhere more appropriate ?
You are correct, John, vibration is an issue, especially with digital cinecams. More appropriate? No, removing the door and filming from the seat has many restrictive limitations, lots of parts getting in the way. The wing mount pod is an ideal solution. Assuming various engineering issues can be sorted. The Hollywood solution involves gimbal mounts and rotorcraft, very expensive stuff, the documentary film we are looking to produce does not have an unlimited budget.
Sounds like you're more interested in just clamping something out there and making do. Without more information and an idea of what you're putting out there, it's hard to be anything but very general on the subject. Also, you really can't separate the safety and the legality; they're tied closely together as both are required in order for the arrangement to be airworthy.
A working unit in the UK, CAA approved, is already in use.
Wildlife, Documentary, Commercial and Aerial Filming: Last Refuge Ltd.
We are, however, unsure of the methods by which the UK CAA approved this unit, no info forthcoming from the manufacturer. The FAA here in the US tends to be more skeptical of such devices. I'm sure, Guppy, you're familiar with that mode. Any thoughts on the subject much appreciated.