Whereas my G9 was obviously designed for both wide angle and telephoto add-on converter lenses, the G12 apparently is not designed for wide angle converter lenses. I say this simply because the G9 menu included a setting for wide angle converter but the G12 menu does not. In addition, the G12 instruction manual includes the Canon TC-DC58D telephoto converter but not the wide angle converter and Canon does not list wide angle converters as accessories for the G12.
On the other hand, Canon does not say "Don't never, ever use a wide angle converter lens with the G12!" and I happen to already have the Canon wide angle converter lens WC-DC58B which is a 0.75x converter. When used with my G9, the WC-DC58B produced a focal length of 26mm (in comparison to full frame 35mm film cameras). With the G12, the same converter lens would produce a focal length of 21mm -- if it worked.
Of course, the WC-DC58B does work with the G12. As shown in the above photo, the 21mme view is quite wider than the natural 28mme of the G12. The inside view is from previous photos of the ski lake and the wider view is with the 21mme attached. (Photo made by superimposing pictures and drawing an outline around the 28mme picture.) The trick to getting the wide angle converter to work is proper mounting on the camera.
Vignetting can be a problem in mounting wide angle converters to the G12. To mount auxiliary lenses and filters to my G12, I use the Lensmate adapter system. In the Lensmate system, the "A" adapter is shorter than simple one piece adapters but is long enough to mount the WC-DC58B wide angle converter lens and also has the proper 58mm threads. To get the above photo, I carefully screwed the converter lens into the adapter while continuously chanting "DO NOT ZOOM! DO NOT ZOOM!". It seems to me that this combination works.
To test the reports of vignetting, I next tried the WC-DC58B on the full Lensmate adapter (A+B components). Yes, vignetting is a problem because the converter lens is not close to the camera lens. With a little zooming, the vignetting disappears but so does the advantage of the wide angle.
Although I've read reports of difficulty in focusing the G12 when the wide angle lens is attached, I've not found focusing to be a problem -- perhaps because I use the small Flexizone frame with my G12 just as I used it with the G9. Of course, depth of field is really deep at 21mme!
On the other hand, reports of focus problems might actually be a result of reduced image quality. The convenience and price is great but, just as in the days of film, these simple screw-on converters have adverse effects on image quality.
My conclusion is that I'll occasionally use the WC-DC58B with the Lensmate "A" adapter on my G12 (DO NOT ZOOM!) for convenience but my preference is to stitch multiple images when the goal is an enlarged print.