I don't have any experience of external light bar fittings, but I do have a bit with mobile radio systems and outdoor floodlighting systems.
From what I can understand you're asking about a way of making the connections to a roof mounted light bar which can be dismounted or fitted as you want to.
If that's the case then the suggestion by Ginge (BGB) would be useful if you have a snorkel, you could run the cable up inside it, the only problem then is what to do with the loose flying socket.
If your plan is to mount a socket on the roof then the considerations will have to be the current carrying capability of the pins, the sealing method needed where the socket is mounted to the bodywork, the sealing method required to prevent water ingress to the plug and socket when the bar is in place and the sealing method when the plug is removed.
I'm not suggesting the you fit a 12s trailer socket on the roof, but if you take a look at the arrangement of the trailer sockets you should get some idea of what I mean.
A socket mounted on the roof will be far more rugged than trying to control a flying lead with a socket on the end of it.
If you intend to permanently fix the light bar in place then there are a whole load of "cable entry glands" available from electrical suppliers.