Hello all.
Now this response will be 5 years too late for some of you, but I've only just stumbled across this discussion. I work for a coaxial connector manufacturer, Coax Connectors Ltd, and last year we developed a cable joiner for just this purpose. The original requirement came from a speed camera manufacturer having repeated issues of their video cables being cut. I can't imagine why anyone would want to do that!
Anyway, we now make the connector on the link below. It is IP68 rated (waterproof) to take into account the instances when the cable joiner may be exposed to the element. So if you do want to do a professional job with minimal losses, this is the way.
http://www.coax-connectors.com/products/connectors/miscellaneous/76-570-q6-ai.aspx
If this is of interest, we'd be more than happy to contact some recommended distributors.
Rgds David