Using the teleconverter is exactly the same as cropping later.
No it isn't. Using a teleconverter maintains image resolution just like when using a longer lens. Where as cropping, you loose resolution. The downside of using a teleconverter vs a dedicated telephoto lens are some IQ loss and/or additional distortion.
This brings me to my question. When wanting to extend the zoom range on the Panasonic I believe there is a tele conversion option that increase the focal range by 1.5x(at 8m pixals) and 2.0x (4m)
AFAIK, Panasonic doesn't make a teleconverter specifically for m43, but there are other options, like the Raynox DCR-1541PRO/DCR-1540PRO
, that can be used on the 45-200mm.
When recording video, the GH2 has an extra-tele video mode where it crops to the center to give 1920x1080 (HD resolution), instead of scaling. But I don't know if this is available on the G3. If it is, a teleconverter is not needed for video.