oh my ...never use a ND filter for protection a MC filter or a skylight filter or most use a Multicoated UV filter, none of which cause any light loss at all.
Well 3/10 of 1% but that almost no loss. And there are several ways to attach a filter without the stupid Panasonic adapter.
One way is to use an after market adapter that is much shorter , but if you do not plan on using any add on lenses a nextphoto adapter is the simplest way.
http://www.nextphoto.net/FZ20.htm
If you do think you may want to add lenses either Telephoto or Wide angle or macro you should look into a 3rd party adapter
either Phayee, lovelifes or the peramaal adapter are all fine. You can perhaps get some more info here and everything about the FZ10 applies to the FZ20 also
http://www.cs.mtu.edu/~shene/DigiCam/