The problem is that tvout encoders can only output certain modelines. Some modern encoders are a bit more flexible but older ones like the chrontell only support a fixed number of modes. In general standard modelines won't work for tvout encoders (and the drivers ignore those; it appeared to work a bit in the past but it didn't really work) If you want to access the chrontell chip yourself you could play with nvtv (http://www.sf.net/projects/nv-tv-out)
. The tool can access the i2c bus of the card and also the tvout encoder.