Unknown coaxial cable female end. smaller than RCA or BNC. White rim
That's a barrel connector, as you say most often used for DC power connections. I doubt that it has an ideal impedance profile for 75 ohm video connections, but that probably doesn't matter in your case.

Get the inner and outer dimensions using a micrometer, match it to one of the standard sizes like type N etc, and you can easily build your own adaptor. I don't think you're going to find anything off the shelf since the use is proprietary / non-standard.