What do you mean by 'custom'?
The Bass.Net Visuals class provides the visuals as mentioned in the docs.
If you want to draw your own visuals, you can do that manually, e.g. by calling BASS_ChannelGetData to get the FFT sample data and do the painting yourself.
well i read read but now see this post :S
http://www.un4seen.com/forum/?topic=14045.0