I think Wallace's suggestion is the solution. Set Flexizone for your AF Frame setting in the Menu, then press the AF Frame Selector on the back of the camera to adjust frame size and location (see pages 88 & 89 in the user guide).
Page 86 of the guide tells you about the half-press for Focus Lock and recompose, but it does not say your camera will not adjust exposure again. I tried it with my G11 and saw the green box indicator, but the box does not change to white on full-press...it simply takes a photo. Looking at the photo taken in Review mode does show a white box in the center of my recomposed image, but I believe the image focus range has not changed.
Page 96 tells about AE lock button, while page 97 tells us about FE (flash exposure) lock if using flash. There are lots of options to use, but figuring out how is another issue.
Another method to lock focus is given in my G9 user guide. It says to half-press to set focus on your subject, then press the MF button, then recompose and complete the full-press. It works on my G11 also, but make sure Safety MF is set to Off in your menu settings.
In the end, Flexizone seems to be the best option.
Dennis