Absolutely. That's why the GameGear can accept a TV Tuner input...It's no more technically advanced that the 2600VCSp that Ben Heckendorm produces!!
The LCD in the Lynx is controlled by rasters and column access, and the display controller is designed as such (IE Column:Offset addressing) from what I remember in the Handy programmers guide. It's theoretically possible to make a video digitiser type interface for the lynx, but without reverse engineering the Mikey chip there's no way to make a direct RGB video interface, that's just not possible.
RE: using lynx or game gear screens