Thanks to all for your responses.
I use a Linhof 4X5 Technika often hand-held (or with a monopod). For head and shoulder portraits with rangefinder focusing I use coupled 210 or 250 mm lenses.
The Broncolor ringflash has a diameter of 200mm, and though attaching it to the Technika would not be a problem for ground-glass focusing, the ringflash would partly cover the rangefinder. This is why I was thinking of using the ringflash off the optical axis of the lens, mounted on the side of the Technika like a flashgun, where it would not impede the use of the rangefinder.
On the other hand, if the ringflash can be mounted a little below and to the left of the centre of the lens axis, the rangefinder would not be obscured and the ringflash would be surrounding the lens...