I've looked for such a tool over the years and have never found one. It's probably because the connector manufacturers don't really want us countering the torque with the relatively fragile bayonet tabs. That, of course, has never stopped me from doing so.
Most of the time I just slide mating connector over the panel mount side of the BNC and the knurl on the mating connector is good enough to crank the mounting nut nice and tight.
When I worked for a company that did satellite earth equipment racks, I took a deep socket (I think it was for a 3/8" hex head) and used a Dremel motor tool with a cutoff wheel to cut opposing notches across the flats (round the edges a bit to keep from marring the bayonets). That allowed easy installation even in tightly spaced panels. You can use either a 1/4" or 3/8" drive socket shaft for it, but I the smaller 1/4" drive socket is easier to modify since it's thinner. Try to find a cheap socket that's made of junkier metal, too, since something like a Snap On or Craftsman will be harder.
Hmmm.... maybe I should set up a manufacturing afcility in my basement to make these.
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster