I'll look up the damper size/measures - it may take some time though; so if I forget - remind me!
As for wrenching out of hand... a damper would help, a lot. But by the description it does sound more like a steeringhead rollerbearing adjustment or wear problem to me...
When you hit sand I guess it will be like hitting a "heap of snow" blown across the road - the snow will grip the frontwheel and do just about what it wants to the bike; there is no way one will be able to hold on... a damper at full damping does have a significant effect here making it just about possible to hold on if the speed is not too high when hitting the snow-"drift(?)".
The wobbel that occurs when one is dropping speed over loose snow is awfull - is that the kind of problem one have on sand too? If so - a damper will help and perhaps even eliminate the problem; however... on sand you are hardly likely to have skiis mounted on the bike are you... -
The load one puts on the skiis ease off the negative load on the fronwheel effectively softening the wobbel, and if that is not enough one moves backwards on the seat as far as one can get (still staning on the skiis) that usually ends deep-snow induced wobbel.
A properly set steeringhead roller bearing will minimize the wobbling /7 frame. I did have all sized up (rusted) rollers... so using chain-lubrication I got them "gliding"... back then I would get wobbel from the white lane separations... (much amuzing to me but horrific to anyone driving behind me).
A worn bear will also cause problems like inducing wobbel...
Then you have the forks... if they start to bend a bit more, ever so littel, than what they were designed for... wobbel.
If you put in stronger springs (like I have K75 springs with 11cm spacer) it too will make the forks unstable...
Then you have the flexibility of the frame - later /7 models looks reinforced to me, but not enough. Luiftmeister did present a lot of neat ideas on where and how to strengthen the /7 frame (street bikes though).
All in all - there are many parts to wringing the handlebar out of ones hands