Switching to macro mode (even if the subject is many feet away!) may help.
Karl Norris (dpreview, Panasonic forum, 8th February 2004)...
With flash `on' and in manual focus mode the EVF of the FZ10 remains bright for focus, and with focus lock obtained {i.e. shutter-release button half-depressed} the flash can be closed without dimming the EVF for framing the image. Now you can {fully} press the shutter to record your photo. This trick can be used for those situations where you don't wish to use the flash, but the dim light makes it difficult to focus and frame your photo. This overcomes the dimming of the EVF if the flash is `OFF' and you try to focus and frame your image. {in later posts, Karl mentions that he is using ISO 50 and a tripod with this trick}