I ask the same question about three years ago, so I am hoping someone might have used one since then and is capable of commenting.

Has anyone ever used a MagnetoSpeed Ballistic Chronograph with their double rifle?

It is barrel mounted; directly strapped on the barrel of the weapon, with the sensor hanging out in front of the gun.

http://www.magnetospeed.com/

All I am looking for is a velocity and not interested in its effect on accuracy.

I used one on my 375 H&H and it worked fine. I recall adjusting it after every shot because of the recoil but that was no big deal. I was testing original Kynoch ammo to get the actual velocity so I could match it with handloads.

I used first generation of Magnetospeed on a .50 DB rifle (indeed, .50 exotic calibre, not a .500) and on 12-bore DB rifles (and a number of SB). I've changed to Labradar in the meantime, but had to use Magnetospeed for the 12-bores as the Labrador got 'confused' by the felt 12-bore wads.
Problem with Magnetospeed is that you have to change between two shots if you'd want to do both barrels. This takes some time which -in theory- could influence precision of the second barrel ...
Beware of taper in barrels !