Come on now, that sounds like a great way to mess up the connector or strip the threads. Modern tools are made that way for a reason, to get a solid connection without wrecking the cable. You might get it to work once by jamming a knife in there, but you are asking for a bad signal or a broken part. Just use the right tool, it is worth it.
Back when I worked AV installs, we sometimes used a flathead for BNC connectors in a bind. It's all about gentle pressure and knowing when to stop so you don't strip anything. Jordan's point stands for daily use, but in a pinch, using whatever's handy can get the job done without wrecking stuff. What's your take on using make-do tools for quick fixes?
Ever notice how this is the same argument as using a butter knife for a screw? Jordan's right about the right tool, but sometimes you just gotta make it work.
You're totally right that the right tool is best for avoiding damage! But in a real pinch, like when you're on a job and something's missing, careful use of a flathead can save the day. It's all about having a steady hand and not forcing it. With some experience, you can tell when to back off before stripping threads. Still, for everyday work, buying the right tools is the way to go!