Jack wrote:
...
What are your suggestions for overcast daylight signalling that do not
involve fire? No doubt some of us would like to ad a trick or two to our
repertoir.
...

In boats, according to the French regulation, it is mandatory to have on
board some devices that generates a very visible orange smoke when fired.
If you feel concerned you can buy one of this things and carry it in your
glider. But maybe the regulation for aircrafts prohibit carrying such
flamable devices.